string.c: rename rb_str_splice as rb_str_update

* string.c (rb_str_update): rename rb_str_splice as exported name
  rb_str_update, and make the former an alias macro of the latter.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45647 b2dd03c8-39d4-4d8f-98ff-823fe69b080e