[PATCH] no table rewrite when set column type to constrained domain

hi.

the attached patch is to implement the $subject feature.
i was mainly intrigued by the discussion in
https://www.postgresql.org/message-id/20190226061450.GA1665944@rfd.leadboat.com

the main gotcha is struct NewColumnValue.
we do ``palloc0(sizeof(NewColumnValue));`` in ATExecAddColumn,
ATExecSetExpression, ATPrepAlterColumnType.

ATExecAddColumn:  Adding a new column with domain with constraints will cause
                  table rewrite.
ATExecSetExpression: for stored generated column will cause table rewrite, we do
                     not support domain over virtual generated columns now.
ATPrepAlterColumnType: we only do table rewriting occasionally.
see ATColumnChangeRequiresRewrite.

If table rewrite is required, then there is nothing we can do. so
we only need to focus on ATPrepAlterColumnType.
we can add a new boolean field, coerce_to_domain, to NewColumnValue. this field
is set to true only when changing an existing column's type to a constrained
domain. In such cases, a table scan is enough—no table rewrite is needed.
coerce_to_domain will set to false, if table rewrite is required.