Re: hiding variable-length fields from Form_pg_* structs

So here is a patch for that.

There are a few cases that break when hiding all variable length fields:

Access to indclass in relcache.c, as discussed upthread, which should be
fixed.

Access to pg_largeobject.data.  This is apparently OK, per comment in
inv_api.c.

Access to pg_proc.proargtypes in various places.  This is clearly
useful, so we'll keep it visible.

So I think the relcache.c thing should be fixed and then this might be
good to go.