__floordiv__ should be part of coercion model #2034
Description
Add __floordiv__ to the coercion model for RingElement and change __floordiv__ to _floordiv_ where applicable.
This does not change any semantics of floor division, but it does fix floor division in a few cases where coercion is involved. It also fixes various segmentation faults like
sage: R.<x> = QQbar[]
sage: int(1) // x
...
Finally, we also implement __itruediv__, which was forgotten when adding __truediv__.
Component: coercion
Author: Jeroen Demeyer
Branch/Commit: 3e0e5b8
Reviewer: Vincent Delecroix
Issue created by migration from https://trac.sagemath.org/ticket/2034