drop string allocations for each resource

Eagerly calculate and cache the name of Symbol objects in the path AST.
This drops about 26 string allocations per resource:

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

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 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:

{:T_SYMBOL=>11}
{:T_REGEXP=>17}
{:T_STRUCT=>6500}
{:T_MATCH=>12004}
{:T_OBJECT=>99009}
{:T_DATA=>116084}
{:T_HASH=>122015}
{:T_STRING=>172647}
{:T_IMEMO=>371132}
{:T_ARRAY=>433056}

After:

{:T_SYMBOL=>11}
{:T_REGEXP=>17}
{:T_STRUCT=>6500}
{:T_MATCH=>12004}
{:T_OBJECT=>99009}
{:T_DATA=>100088}
{:T_HASH=>122015}
{:T_STRING=>159637}
{:T_IMEMO=>363134}
{:T_ARRAY=>433056}
```

0 files changed, 0 insertions, 0 deletions