* NEWS: Add note about frozen string literals

* compile.c (case_when_optimizable_literal): optimize NODE_LIT strings
  in when clauses of case statements

* ext/ripper/eventids2.c: add tSTRING_SUFFIX

* parse.y: add 'f' suffix on string literals for frozen strings

* test/ripper/test_scanner_events.rb: add scanner tests

* test/ruby/test_string.rb: add frozen string tests

[Feature #8579] [ruby-core:55699]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42773 b2dd03c8-39d4-4d8f-98ff-823fe69b080e