The previous implementation of `ModularInverse` was flaky and did not
compute the correct value in many occasions, especially with big numbers
like in RSA.
Also added a bunch of tests with big numbers.
The trimmed cache length of the `UnsignedBigInteger` was not reset after
an `add_into_accumulator_without_allocation` operation because the
function manipulates the words directly.
This meant that if the trimmed length was calculated before this
operation it would be wrong after.
