Improve efficiency of divmod helper method get_max_shift()

[saxbophone]

This loop was introduced into this method in #107 to fix a bug, but constitutes a potential performance bottleneck due to the repeated multiplication in the loop predicate:

arby/arby/include/arby/Nat.hpp

Lines 534 to 537 in ca58d11

	// drag back down wiggle_room while shifted rhs > lhs
	while (rhs * shift > lhs) {
	shift._digits.pop_back();
	}