div with rounding modes [+ rounded division] (#33040)
* Re-arrange fld/cld code
In preparation for supporting other rounding modes in div, create
a three-argument div function that takes a rounding mode as the last
argument and make this the fundamental fallback for fld/cld.
* Implemented rounded division
* add various divrem combinations to avoid overflow
* Whitespace/test fixes
* Small tweaks to docstrings
* Bugfixes
* Add the exhaustive test
* Tigthen up types for div fallback
I think it's better to give a MethodError here than an approximate
answer for non-AbstractFloat reals (e.g. custom integer types).
