Speed up partial rendering by caching "variable" calculation

This commit speeds up rendering partials by caching the variable name
calculation on the template.  The variable name is based on the "virtual
path" used for looking up the template.  The same virtual path
information lives on the template, so we can just ask the cached
template object for the variable.

This benchmark takes a couple files, so I'll cat them below:

```
[aaron@TC ~/g/r/actionview (speed-up-partials)]$ cat render_benchmark.rb
require "benchmark/ips"
require "action_view"
require "action_pack"
require "action_controller"

class TestController < ActionController::Base
end

TestController.view_paths = [File.expand_path("test/benchmarks")]
controller_view = TestController.new.view_context

result = Benchmark.ips do |x|
  x.report("render") do
    controller_view.render("many_partials")
  end
end
[aaron@TC ~/g/r/actionview (speed-up-partials)]$ cat test/benchmarks/test/_many_partials.html.erb
Looping:
<ul>
<% 100.times do |i| %>
  <%= render partial: "list_item", locals: { i: i } %>
<% end %>
</ul>
[aaron@TC ~/g/r/actionview (speed-up-partials)]$ cat test/benchmarks/test/_list_item.html.erb
<li>Number: <%= i %></li>
```

Benchmark results (master):

```
[aaron@TC ~/g/r/actionview (master)]$ be ruby render_benchmark.rb
Warming up --------------------------------------
              render    41.000  i/100ms
Calculating -------------------------------------
              render    424.269  (± 3.5%) i/s -      2.132k in   5.031455s
```

Benchmark results (this branch):

```
[aaron@TC ~/g/r/actionview (speed-up-partials)]$ be ruby render_benchmark.rb
Warming up --------------------------------------
              render    50.000  i/100ms
Calculating -------------------------------------
              render    521.862  (± 3.8%) i/s -      2.650k in   5.085885s
```

3 files changed, 41 insertions, 19 deletions

diff --git a/actionview/lib/action_view/renderer/partial_renderer.rb b/actionview/lib/action_view/renderer/partial_renderer.rb
index 478400e016..801916954f 100644
--- a/actionview/lib/action_view/renderer/partial_renderer.rb
+++ b/actionview/lib/action_view/renderer/partial_renderer.rb
@@ -295,8 +295,21 @@ module ActionView
     end
 
     def render(context, options, block)
-      setup(context, options, block)
-      template = find_partial
+      as = as_variable(options)
+      setup(context, options, as, block)
+
+      if @path
+        if @has_object || @collection
+          @variable, @variable_counter, @variable_iteration = retrieve_variable(@path, as)
+          @template_keys = retrieve_template_keys(@variable)
+        else
+          @template_keys = @locals.keys
+        end
+        template = find_partial(@path, @template_keys)
+        @variable ||= template.variable
+      else
+        template = nil
+      end
 
       @lookup_context.rendered_format ||= begin
         if template && template.formats.first
@@ -359,7 +372,7 @@ module ActionView
       # If +options[:partial]+ is a string, then the <tt>@path</tt> instance variable is
       # set to that string. Otherwise, the +options[:partial]+ object must
       # respond to +to_partial_path+ in order to setup the path.
-      def setup(context, options, block)
+      def setup(context, options, as, block)
         @options = options
         @block   = block
 
@@ -382,25 +395,25 @@ module ActionView
 
           if @collection
             paths = @collection_data = @collection.map { |o| partial_path(o, context) }
-            @path = paths.uniq.one? ? paths.first : nil
+            if paths.uniq.length == 1
+              @path = paths.first
+            else
+              paths.map! { |path| retrieve_variable(path, as).unshift(path) }
+              @path = nil
+            end
           else
             @path = partial_path(@object, context)
           end
         end
 
+        self
+      end
+
+      def as_variable(options)
         if as = options[:as]
           raise_invalid_option_as(as) unless /\A[a-z_]\w*\z/.match?(as.to_s)
-          as = as.to_sym
+          as.to_sym
         end
-
-        if @path
-          @variable, @variable_counter, @variable_iteration = retrieve_variable(@path, as)
-          @template_keys = retrieve_template_keys
-        else
-          paths.map! { |path| retrieve_variable(path, as).unshift(path) }
-        end
-
-        self
       end
 
       def collection_from_options
@@ -414,8 +427,8 @@ module ActionView
         @object.to_ary if @object.respond_to?(:to_ary)
       end
 
-      def find_partial
-        find_template(@path, @template_keys) if @path
+      def find_partial(path, template_keys)
+        find_template(path, template_keys)
       end
 
       def find_template(path, locals)
@@ -511,9 +524,9 @@ module ActionView
         end
       end
 
-      def retrieve_template_keys
+      def retrieve_template_keys(variable)
         keys = @locals.keys
-        keys << @variable if @has_object || @collection
+        keys << variable
         if @collection
           keys << @variable_counter
           keys << @variable_iteration
diff --git a/actionview/lib/action_view/template.rb b/actionview/lib/action_view/template.rb
index 3b2c264ed4..8a5407c622 100644
--- a/actionview/lib/action_view/template.rb
+++ b/actionview/lib/action_view/template.rb
@@ -128,6 +128,8 @@ module ActionView
       end
     end
 
+    attr_reader :variable
+
     def initialize(source, identifier, handler, details)
       format = details[:format] || (handler.default_format if handler.respond_to?(:default_format))
 
@@ -138,6 +140,13 @@ module ActionView
       @original_encoding = nil
       @locals            = details[:locals] || []
       @virtual_path      = details[:virtual_path]
+
+      @variable = if @virtual_path
+        base = @virtual_path[-1] == "/" ? "" : File.basename(@virtual_path)
+        base =~ /\A_?(.*?)(?:\.\w+)*\z/
+        $1.to_sym
+      end
+
       @updated_at        = details[:updated_at] || Time.now
       @formats           = Array(format).map { |f| f.respond_to?(:ref) ? f.ref : f  }
       @variants          = [details[:variant]]
diff --git a/activerecord/lib/active_record/railties/collection_cache_association_loading.rb b/activerecord/lib/active_record/railties/collection_cache_association_loading.rb
index dfaac4eefb..d57680aaaa 100644
--- a/activerecord/lib/active_record/railties/collection_cache_association_loading.rb
+++ b/activerecord/lib/active_record/railties/collection_cache_association_loading.rb
@@ -3,7 +3,7 @@
 module ActiveRecord
   module Railties # :nodoc:
     module CollectionCacheAssociationLoading #:nodoc:
-      def setup(context, options, block)
+      def setup(context, options, as, block)
         @relation = relation_from_options(options)
 
         super