bpo-29753: fix merging packed bitfields in ctypes struct/union (GH-19850)

From the commit message:

> When the structure is packed we should always expand when needed,
> otherwise we will add some padding between the fields. This patch makes
> sure we always merge bitfields together. It also changes the field merging
> algorithm so that it handles bitfields correctly.

Automerge-Triggered-By: GH:jaraco