string.c: rb_fstring_new assumes US-ASCII

* string.c (rb_fstring_new): make US-ASCII string.  another
  function may be used for non-ASCII strings.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46689 b2dd03c8-39d4-4d8f-98ff-823fe69b080e