polys: change docstirings to be 'raw' strings in modulargcd
polys: rewrite _swap function in modulargcd

No more conversions to dense representation are used for this function.
polys: some minor changes in modulargcd

In modgcd_multivariate: 
The computation of the bounds is now faster and more readable.

In _modgcd_multivariate_p:
Checking if the result of the recursive call is a constant
is now done with `ha.is_ground` instead of `ha.LM == zero`.
polys: choose evaluation points at random in _modgcd_multivariate_p

and add a test for this