Move renametrig() from tablecmds.c to trigger.c --- if we're going to

Move renametrig() from tablecmds.c to trigger.c --- if we're going to
divide backend/commands by object type, let's try to pay at least
minimal attention to respecting that structure, eh?  Also reorder the
contents of tablecmds.c; it seems odd to me to put ALTER commands before
creation/deletion commands.