Speed up homogeneous AR lists / reduce allocations

This commit speeds up allocating homogeneous lists of AR objects.  We
can know if the result set contains an STI column before initializing
every AR object, so this change pulls the "does this result set contain
an STI column?" test up, then uses a specialized instantiation function.
This way we only have to check for an STI column once rather than N
times.

This change also introduces a new initialization function that is meant
for use when allocating AR objects that come from the database.  Doing
this allows us to eliminate one hash allocation per AR instance.

Here is a benchmark:

```ruby
require 'active_record'
require 'benchmark/ips'

ActiveRecord::Base.establish_connection adapter: "sqlite3", database: ":memory:"

ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name
    t.timestamps null: false
  end
end

class User < ActiveRecord::Base; end

2000.times do
  User.create!(name: "Gorby")
end

Benchmark.ips do |x|
  x.report("find") do
    User.limit(2000).to_a
  end
end
```

Results:

Before:

```
[aaron@TC activerecord (master)]$ be ruby -I lib:~/git/allocation_tracer/lib speed.rb
Warming up --------------------------------------
                find     5.000  i/100ms
Calculating -------------------------------------
                find     56.192  (± 3.6%) i/s -    285.000  in   5.080940s
```

After:

```
[aaron@TC activerecord (homogeneous-allocation)]$ be ruby -I lib:~/git/allocation_tracer/lib speed.rb
Warming up --------------------------------------
                find     7.000  i/100ms
Calculating -------------------------------------
                find     72.204  (± 2.8%) i/s -    364.000  in   5.044592s
```

1 files changed, 20 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index e1a0b2ecf8..9c9fc77a0d 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -350,6 +350,26 @@ module ActiveRecord
     end
 
     ##
+    # Initializer used for instantiating objects that have been read from the
+    # database.  +attributes+ should be an attributes object, and unlike the
+    # `initialize` method, no assignment calls are made per attribute.
+    #
+    # :nodoc:
+    def init_from_db(attributes)
+      init_internals
+
+      @new_record = false
+      @attributes = attributes
+
+      yield self if block_given?
+
+      _run_find_callbacks
+      _run_initialize_callbacks
+
+      self
+    end
+
+    ##
     # :method: clone
     # Identical to Ruby's clone method.  This is a "shallow" copy.  Be warned that your attributes are not copied.
     # That means that modifying attributes of the clone will modify the original, since they will both point to the