Add infrastructure to support a hierarchy of memory allocations with
automatic deallocation of substructure.
Additional malloc sanity changes.  Use sqlite3MemLink() on Index.zColAff and
Table.zColAff as a proof of concept.
Further examples of using automatic deallocation to replace "delete" methods.
Fixes to prior checkins so that they compile and run even if SQLITE_MEMDEBUG
is not defined.