This helps make callers only use the slice of the output buffer that
was written to.
As part of updating the callers of the API several bugs were fixed and
useless code paths were removed:
- The exported data is not host-endianess dependent (always big endian)
- The exported data does not contain leading zeros
- The output buffer is only written up to the result's size
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.
