The code in rewriteDefine.c didn't know about AO tables, and tried to use
heap_beginscan() on an AO table. It resulted in an ERROR, because even for
an empty table, RelationGetNumberOfBlocks() returned 1, but we tried to
scan it like a heap table, heap_getnext() couldn't find the block. To fix,
add an explicit check that you can only turn heap tables into views this
way. This also forbids turning an external table into a view. (We could
possible relax that limitation, but it's a pointless feature anyway we only
support for backwards-compatibility reasons in PostgreSQL.)

Also add test case for this.