Allow use of table rowtypes directly as column types of other tables.

Allow use of table rowtypes directly as column types of other tables.
Instead of prohibiting that, put code into ALTER TABLE to reject ALTERs
that would affect other tables' columns.  Eventually we will probably
want to extend ALTER TABLE to actually do something useful here, but
in the meantime it seems wrong to forbid the feature completely just
because ALTER isn't fully baked.