* Add an optimized squaring routine under the `sqr` name.
  Algorithms for squaring bigger numbers efficiently will come in a
  PR later.
* Fix a bug where a multiplication was done twice if the threshold for
  the use of Karatsuba algorithm was crossed. Add a test to make sure
  this won't happen again.
* Streamline `pow` method, take a `Const` parameter.
* Minor tweaks to `pow`, avoid bit-reversing the exponent.