Reserve the FL_USER18 bit for RSTRING_HTML_SAFE

I want to play with implementing `String#html_safe?` and friends in a C extension that stores this boolean as a bit on the `RString` struct rather than as an ivar.

I think this will make quite a difference, as instance variables on non-RObject objects in Ruby require two hashtable lookups per get/set.

cc @github/mri-hackers