[EXPERIMENTAL] Make Symbol#to_s return a frozen String
[EXPERIMENTAL] Make Symbol#to_s return a frozen String

* Always the same frozen String for a given Symbol.
* Avoids extra allocations whenever calling Symbol#to_s.
* See [Feature #16150]

ruby/spec,nobu/rubyspec,nobu/rubyspec,ruby/spec,eregon/rubyspec,eregon/rubyspec,eregon/rubyspec,ruby/spec,nobu/rubyspec