If they use up so much stack space, contain (sometimes several) loops, and take
a noticable amount of time anyway, then 'inline' is probably going to be ignored
by the compiler anyway.
This patchset adds a simple SignedBigInteger that is entirely defined in
terms of UnsignedBigInteger.
It also adds a NumberTheory::Power function, which is terribly
inefficient, but since the use of exponentiation is very much
discouraged for large inputs, no particular attempts were made
to make it more performant.
