Make useful infrastructure from aset.c generally available.

An upcoming patch introduces a new type of memory context. To avoid
duplicating debugging infrastructure within aset.c, move useful pieces
to memdebug.[ch].

While touching aset.c, fix printf format code in AllocFree* debug
macros.

Author: Tomas Vondra
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/b3b2245c-b37a-e1e5-ebc4-857c914bc747@2ndquadrant.com