string.c: move frozen_strings table to rb_vm_t

Cleanup in case MVM development proceeds.

* string.c: remove static frozen_strings
* string.c (Init_frozen_strings): new function
* string.c (rb_fstring): remove check for frozen strings,
  use per-VM table
* string.c (rb_str_free): use per-VM table
* string.c (Init_String): use per-VM table
* vm_core.h (rb_vm_t): add frozen_strings table
* internal.h (Init_frozen_strings): new function prototype
* eval.c (ruby_setup): call Init_frozen_strings
  [Feature #10182]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47310 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-08-29

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47311 b2dd03c8-39d4-4d8f-98ff-823fe69b080e