implement column._merge()

implement column._merge()

this takes the user-defined args of one Column and merges
them into the not-user-defined args of another Column.

Implemented within the pep-593 column transfer operation
to begin to make this new feature more robust.

work may still be needed for constraints etc. but
in theory everything from the left side annotated column
should take effect for the right side if not otherwise
specified on the right.

Change-Id: I57eb37ed6ceb4b60979a35cfc4b63731d990911d