Fast conversion

[make-github-pseudonymous-again]

Conversion can still be fast when bases have a common power. A bit of shifting needs to be done but this is pretty similar to the fast conversion methods already implemented.

[make-github-pseudonymous-again]

Seems difficult to test for that... We could restrict to powers of two only.

[make-github-pseudonymous-again]

Testing for powers of two can be done very fast with (f & (f-1)) && (t & (t-1)). Then number of bits per limb can be found with https://stackoverflow.com/a/7813486/1582182.