Feature #7035

* compile.c (defined_expr), insns.def (defined): share single frozen
  strings.  [EXPERIMENTAL] [ruby-core:47558][Feature #7035]
* iseq.c (rb_iseq_defined_string): make expression strings.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37025 b2dd03c8-39d4-4d8f-98ff-823fe69b080e