Add deprecation warning to bad uses of lit()

[captdeaf]

I'd really like to rewrite the parser and make its uses cleaner. FN_LITERAL, which is only used by lit(), is a big pain in my posterior because of how it alters the parser.

I'd like to switch lit() from FN_LITERAL to FN_NOPARSE. This PR is a first step - if lit() is called with mismatched, unescaped {}s or []s, this will simply warn executor and enactor.

[talvo]

This is still a no from me, even if you write it as a PR instead of a ticket :P