Remove ORM elements from annotations at the schema level.

Remove ORM elements from annotations at the schema level.

Fixed issue where when constructing constraints from ORM-bound columns,
primarily :class:`.ForeignKey` objects but also :class:`.UniqueConstraint`,
:class:`.CheckConstraint` and others, the ORM-level
:class:`.InstrumentedAttribute` is discarded entirely, and all ORM-level
annotations from the columns are removed; this is so that the constraints
are still fully pickleable without the ORM-level entities being pulled in.
These annotations are not necessary to be present at the schema/metadata
level.

Fully implemented coercions for constraint columns within
schema.py, including for FK referenced columns.

Fixes: #5001
Change-Id: I895400dd979310be034085d207f096707c635909