Resolve "Rewrite bmath to allow type hints and attribute proposals"

The bmath backend is rewritten to allow linters proposing the functions available in bmath. Additionally, this gives a better overview on which backend lacks which function. Type checkers will also be able to complain about wrong typing of bmath functions.

Known issues:

import blond.utils.bmath as bm

will not work anymore and must be replaced by

from blond.utils import bmath as bm

This is because bmath is not a file anymore, but a class inside utils.