string.c: remove vm_core.h dependency [ruby-core:64627]

* string.c: revert part of r47311, add rb_vm_fstring_table(),
  remove vm_core.h dependency. [ruby-core:64627]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47312 b2dd03c8-39d4-4d8f-98ff-823fe69b080e