drop object allocation during routes setup

This commit introduces a functional Path AST visitor and implements
`each` on the AST in terms of the functional visitor.  The functional
visitor doesn't maintain state, so we only need to allocate one of them.

Given this benchmark route file:

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

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

ObjectSpace::AllocationTracer.setup(%i{path line type})

result = ObjectSpace::AllocationTracer.trace do
  500.times{|i|
    routes.resource :omglol
  }
end

result.find_all { |k,v| k.first =~ /git\/rails/ }.sort_by { |k,v|
  v.first
}.each { |k,v|
  p k => v
}
```

node.rb line 17 was in our top 3 allocation spot:

```
{["/Users/aaron/git/rails/actionpack/lib/action_dispatch/journey/nodes/node.rb", 17, :T_OBJECT]=>[31526, 0, 28329, 0, 2, 1123160]}
{["/Users/aaron/git/rails/actionpack/lib/action_dispatch/routing/mapper.rb", 2080, :T_IMEMO]=>[34002, 0, 30563, 0, 2, 1211480]}
{["/Users/aaron/git/rails/actionpack/lib/action_dispatch/routing/mapper.rb", 2071, :T_IMEMO]=>[121934, 1, 109608, 0, 7, 4344400]}
```

This commit eliminates allocations at that place.

1 files changed, 3 insertions, 3 deletions

diff --git a/actionpack/lib/action_dispatch/journey/nodes/node.rb b/actionpack/lib/action_dispatch/journey/nodes/node.rb
index 2b57c368dc..542e8c6577 100644
--- a/actionpack/lib/action_dispatch/journey/nodes/node.rb
+++ b/actionpack/lib/action_dispatch/journey/nodes/node.rb
@@ -14,15 +14,15 @@ module ActionDispatch
         end
 
         def each(&block)
-          Visitors::Each.new(block).accept(self)
+          Visitors::Each::INSTANCE.accept(self, block)
         end
 
         def to_s
-          Visitors::String.new.accept(self)
+          Visitors::String::INSTANCE.accept(self, '')
         end
 
         def to_dot
-          Visitors::Dot.new.accept(self)
+          Visitors::Dot::INSTANCE.accept(self)
         end
 
         def to_sym