use the strategy pattern to match request verbs

Rather than building a regexp for every route, lets use the strategy
pattern to select among objects that can match HTTP verbs.  This commit
introduces strategy objects for each verb that has a predicate method on
the request object like `get?`, `post?`, etc.

When we build the route object, look up the strategy for the verbs the
user specified.  If we can't find it, fall back on string matching.

Using a strategy / null object pattern (the `All` VerbMatcher is our
"null" object in this case) we can:

1) Remove conditionals
2) Drop boot time allocations
2) Drop run time allocations
3) Improve runtime performance

Here is our boot time allocation benchmark:

```ruby
require 'action_pack'
require 'action_dispatch'

route_set = ActionDispatch::Routing::RouteSet.new
routes = ActionDispatch::Routing::Mapper.new route_set

result = ObjectSpace::AllocationTracer.trace do
  500.times do
    routes.resources :foo
  end
end

sorted = ObjectSpace::AllocationTracer.allocated_count_table.sort_by(&:last)
sorted.each do |k,v|
  next if v == 0
  p k => v
end
__END__

Before:

$ be ruby -rallocation_tracer route_test.rb
{:T_SYMBOL=>11}
{:T_REGEXP=>4017}
{:T_STRUCT=>6500}
{:T_MATCH=>12004}
{:T_DATA=>84092}
{:T_OBJECT=>99009}
{:T_HASH=>122015}
{:T_STRING=>216652}
{:T_IMEMO=>355137}
{:T_ARRAY=>441057}

After:

$ be ruby -rallocation_tracer route_test.rb
{:T_SYMBOL=>11}
{:T_REGEXP=>17}
{:T_STRUCT=>6500}
{:T_MATCH=>12004}
{:T_DATA=>84092}
{:T_OBJECT=>99009}
{:T_HASH=>122015}
{:T_STRING=>172647}
{:T_IMEMO=>355136}
{:T_ARRAY=>433056}
```

This benchmark adds 500 resources. Each resource has 8 routes, so it
adds 4000 routes.  You can see from the results that this patch
eliminates 4000 Regexp allocations, ~44000 String allocations, and ~8000
Array allocations.  With that, we can figure out that the previous code
would allocate 1 regexp, 11 strings, and 2 arrays per route *more* than
this patch in order to handle verb matching.

Next lets look at runtime allocations:

```ruby
require 'action_pack'
require 'action_dispatch'
require 'benchmark/ips'

route_set = ActionDispatch::Routing::RouteSet.new
routes = ActionDispatch::Routing::Mapper.new route_set

routes.resources :foo

route = route_set.routes.first
request = ActionDispatch::Request.new("REQUEST_METHOD" => "GET")

result = ObjectSpace::AllocationTracer.trace do
  500.times do
    route.matches? request
  end
end

sorted = ObjectSpace::AllocationTracer.allocated_count_table.sort_by(&:last)
sorted.each do |k,v|
  next if v == 0
  p k => v
end

__END__

Before:

$ be ruby -rallocation_tracer route_test.rb
{:T_MATCH=>500}
{:T_STRING=>501}
{:T_IMEMO=>1501}

After:

$ be ruby -rallocation_tracer route_test.rb
{:T_IMEMO=>1001}
```

This benchmark runs 500 calls against the `matches?` method on the route
object.  We check this method in the case that there are two methods
that match the same path, but they are differentiated by the verb (or
other conditionals).  For example `POST /users` vs `GET /users`, same
path, different action.

Previously, we were using regexps to match against the verb.  You can
see that doing the regexp match would allocate 1 match object and 1
string object each time it was called.  This patch eliminates those
allocations.

Next lets look at runtime performance.

```ruby
require 'action_pack'
require 'action_dispatch'
require 'benchmark/ips'

route_set = ActionDispatch::Routing::RouteSet.new
routes = ActionDispatch::Routing::Mapper.new route_set

routes.resources :foo

route = route_set.routes.first
match = ActionDispatch::Request.new("REQUEST_METHOD" => "GET")
no_match = ActionDispatch::Request.new("REQUEST_METHOD" => "POST")

Benchmark.ips do |x|
  x.report("match") do
    route.matches? match
  end

  x.report("no match") do
    route.matches? no_match
  end
end

__END__

Before:

$ be ruby -rallocation_tracer runtime.rb
Calculating -------------------------------------
               match    17.145k i/100ms
            no match    24.244k i/100ms
-------------------------------------------------
               match    259.708k (± 4.3%) i/s -      1.303M
            no match    453.376k (± 5.9%) i/s -      2.279M

After:

$ be ruby -rallocation_tracer runtime.rb
Calculating -------------------------------------
               match    23.958k i/100ms
            no match    29.402k i/100ms
-------------------------------------------------
               match    465.063k (± 3.8%) i/s -      2.324M
            no match    691.956k (± 4.5%) i/s -      3.469M

```

This tests tries to see how many times it can match a request per
second.  Switching to method calls and string comparison makes the
successful match case about 79% faster, and the unsuccessful case about
52% faster.

That was fun!

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