Made several changes to improve MATCH-CASE (following a note from Ivan

Made several changes to improve MATCH-CASE (following a note from Ivan
Boldyrev from a long time ago), MATCHING and MATCH.

Else-clauses are now handled correctly (AFAICT).

Single variable templates in MATCH, MATCH-CASE and MATCHING clauses do
not need to be quoted.

MATCHING was generating one gensym'ed variable per clause without
creating an appropriate enclosing LET. This is now fixed.