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.
This change leads to between 10% and 35% performance improvement when executing
the RSA decryption method.
The main impact is to drastically reduce the number of allocations done in this
method from around 50% of the profile hits to less than 2%.
