Speed up partial rendering by caching "variable" calculation

[tenderlove]

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

Caching is enabled, so this improvement should impact production apps. Also, the logic in the partial renderer is pretty complex, we may not even need that value to be calculated.

[rafaelfranca]

this as = is not necessary here anymore.

[tenderlove]

nice catch, thanks!