Remove extra ast pass for Pattern#names

Before this commit #ast and #names were each doing a separate pass over
the journey ast. Since both of these methods are called for every route
definition, we can do that work up front in #initialize, consolidating
into a single pass through the ast.

I think we can also get rid of the `spec` alias, but I am planning to do
that in a separate PR to limit the scope of this one.

This is similar to the work done in rails#38901 and rails#39903, and the benchmark
results are similar:

```
Warming up --------------------------------------
              before     7.791k i/100ms
               after    15.843k i/100ms
Calculating -------------------------------------
              before     78.081k (± 1.4%) i/s -    397.341k in   5.089797s
               after    159.093k (± 1.8%) i/s -    807.993k in   5.080367s

Comparison:
               after:   159093.1 i/s
              before:    78081.2 i/s - 2.04x  (± 0.00) slower

MEMORY
Calculating -------------------------------------
              before   240.000  memsize (    40.000  retained)
                         4.000  objects (     1.000  retained)
                         0.000  strings (     0.000  retained)
               after   120.000  memsize (    40.000  retained)
                         2.000  objects (     1.000  retained)
                         0.000  strings (     0.000  retained)

Comparison:
               after:        120 allocated
              before:        240 allocated - 2.00x more
```

actionpack/lib/action_dispatch/journey/path/pattern.rb

@@ -4,7 +4,8 @@ module ActionDispatch
   module Journey # :nodoc:
     module Path # :nodoc:
       class Pattern # :nodoc:
-        attr_reader :spec, :requirements, :anchored
+        attr_reader :names, :spec, :requirements, :anchored
+        alias :ast :spec
 
         def self.from_string(string)
           build(string, {}, "/.?", true)
@@ -22,40 +23,33 @@ def initialize(ast, requirements, separators, anchored)
           @separators   = separators
           @anchored     = anchored
 
-          @names          = nil
           @optional_names = nil
           @required_names = nil
           @re             = nil
           @offsets        = nil
-        end
-
-        def build_formatter
-          Visitors::FormatBuilder.new.accept(spec)
-        end
-
-        def eager_load!
-          required_names
-          offsets
-          to_regexp
-          nil
-        end
 
-        def ast
+          @names = []
           @spec.each do |node|
             if node.symbol?
+              @names << node.name
               re = @requirements[node.to_sym]
               node.regexp = re if re
             elsif node.star?
               node = node.left
               node.regexp = @requirements[node.to_sym] || /(.+)/
             end
           end
+        end
 
-          @spec
+        def build_formatter
+          Visitors::FormatBuilder.new.accept(spec)
         end
 
-        def names
-          @names ||= spec.find_all(&:symbol?).map(&:name)
+        def eager_load!
+          required_names
+          offsets
+          to_regexp
+          nil
         end
 
         def required_names