Parsing ambiguity in recent NOT DEFERRABLE clause

Refer to the new tests for the ambiguous case. Prior to this branch, 8b gets a parse error.

This seems like a case where a numeric lookahead is more appropriate than the fairly complicated syntactic lookahead, even though the parser doesn't have a lot of those currently.

Note that NOT DEFERRABLE is the default, so one doesn't actually need to say it, but it's standard.