string.c: rb_fstring_new

* string.c (rb_fstring_new): create fstring from pointer and
  length.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46626 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
gc.c: freeze GC::OPTS

* gc.c (Init_GC): freeze GC::OPTS.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46627 b2dd03c8-39d4-4d8f-98ff-823fe69b080e