Replace the implementation of maths in `UnsignedBigInteger`
and `SignedBigInteger` with LibTomMath. This gives benefits in terms of
less code to maintain, correctness and speed.
These changes also remove now-unsued methods and improve the error
propagation for functions allocating lots of memory. Additionally, the
new implementation is always trimmed and won't have dangling zeros when
exporting it.
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