Chez Scheme GC: faster record type handling
Chez Scheme GC: faster record type handling

When the GC needs to copy/mark a record, it previously forced a
copy/mark on the record's type descriptor, since the copy/mark needs
information from the descriptor. But the needed information is not in
danger of being overwritten for forwading (since it's after the first
two words of the type descriptor), so it's ok to use the old reference
as-is --- at least in non-counting mode. Simplifying record-type
handling and deferring the record-type update to the sweep phase, the
same as for other components of the record type, makes the GC slightly
faster.

mflatt/ChezScheme,mflatt/ChezScheme,mflatt/ChezScheme