Extend the MinimalTuple concept to tuplesort.c, thereby reducing the

per-tuple space overhead for sorts in memory.  I chose to replace the
previous patch that tried to write out the bare minimum amount of data
when sorting on disk; instead, just dump the MinimalTuples as-is.  This
wastes 3 to 10 bytes per tuple depending on architecture and null-bitmap
length, but the simplification in the writetup/readtup routines seems
worth it.