Fix definition of full-match? (Proposed fix for #816)

[dpk]

Full matching applies for all syntax rules if any of the syntax patterns is not a pair. Maintains the old behaviour for conformant R7RS syntax-rules macros (without breaking the count-to-2_ test), but also allows identifier macros, and fully fixes identifier-syntax.

[ashinn]

Thanks Daphne, this is simpler than what I had in mind. I guess it won't work if they try to write a set! macro without ref support, but nobody would do that. Let me take a look.