Herbert Valerio Riedel authored Oct 16, 2017 and Ben Gamari committed Oct 16, 2017

The default implementation of `clearBit` is in terms of
`complement`. However, `complement` is not well-defined
for `Natural` and this consequently renders the default
implementation of `clearBit` dysfunctional.

This implements `clearBit` in terms of `testBit`
and `setBit` which are both well-defined for `Natural`s.

This addresses #13203

(cherry picked from commit 5984a698)