Reduce allocations and retentions in Journey TransitionTable

`DEFAULT_EXP` is by far the most common regexp being anchored.
By special casing it, we can re-use the existing `DEFAULT_EXP_ANCHORED`
regexp, and avoid keeping lots of copies of it in memory. Every time
we hit, we also avoid having to compile the regexp, so it's faster.

We also replace the `Regexp.new(/\A#{sym}\Z/)` pattern by just
`/\A#{sym}\Z/`, as it was compiling and instantiating an extra
regexp for no good reason.

actionpack/lib/action_dispatch/journey/gtg/transition_table.rb

@@ -11,7 +11,7 @@ class TransitionTable # :nodoc:
         attr_reader :memos
 
         DEFAULT_EXP = /[^.\/?]+/
-        DEFAULT_EXP_ANCHORED = Regexp.new(/\A#{DEFAULT_EXP}\Z/)
+        DEFAULT_EXP_ANCHORED = /\A#{DEFAULT_EXP}\Z/
 
         def initialize
           @stdparam_states = {}
@@ -165,7 +165,11 @@ def []=(from, to, sym)
           case sym
           when Regexp
             # we must match the whole string to a token boundary
-            sym = Regexp.new(/\A#{sym}\Z/)
+            if sym == DEFAULT_EXP
+              sym = DEFAULT_EXP_ANCHORED
+            else
+              sym = /\A#{sym}\Z/
+            end
           when Symbol
             # account for symbols in the constraints the same as strings
             sym = sym.to_s