added ALTER TABLE DROP COLUMN, early version

src/backend/access/heap/heapam.c

src/backend/catalog/catalog.c

src/backend/catalog/heap.c

src/backend/catalog/pg_type.c

src/backend/commands/command.c

src/backend/commands/rename.c

src/backend/parser/gram.y

src/backend/utils/adt/name.c

src/backend/utils/cache/relcache.c

src/backend/utils/cache/temprel.c

src/bin/psql/command.c

src/include/access/heapam.h

src/include/catalog/heap.h

src/include/catalog/pg_type.h

src/include/commands/rename.h

src/include/utils/builtins.h

src/include/utils/rel.h

src/include/utils/relcache.h

src/include/utils/temprel.h