allow user-defined STRING synonyms in MAKE-SEQUENCE
...and MAP, MERGE, COERCE and CONCATENATE too. This also meant
working a bit on TYPEXPAND, to make (STRING <N>) be unexpanded in the
same way as STRING, and consequently on VALUES-SPECIFIER-TYPE to
ensure that the system would still recognize those types.
Include test cases for the sequence functions but not for TYPEXPAND.