Add in support for hex literals using underscores

[rv32ima]

Adds in support for hex literals with underscores for better readability. Should not change the existing behavior, but added several unit-tests to verify that the underscore literals are equivalent to non-underscore literals.

Note: in static BigUIntView fromHexString(), we actually over-allocate for the amount of coefficients that we need

mir-algorithm/source/mir/bignum/low_level_view.d

Line 572 in 085212a

auto length = str.length / (W.sizeof * 2) + (str.length % (W.sizeof * 2) != 0);

This is sliced down to the amount that was actually taken here (where j represents the number of non-underscore characters)

mir-algorithm/source/mir/bignum/low_level_view.d

Line 680 in 085212a

coefficients = coefficients[0 .. (j / (W.sizeof * 2) + (j % (W.sizeof * 2) != 0))];

This does increase the initial memory footprint (especially as the input string gets longer), but I haven't been able to think of a good way to get around this while keeping fromHexStringImpl @nogc. Ideally, I want to avoid iterating through the whole string twice, but that seems like the only solution that seems feasible to me.

[9il]

This does increase the initial memory footprint (especially as the input string gets longer), but I haven't been able to think of a good way to get around this while keeping fromHexStringImpl @nogc. Ideally, I want to avoid iterating through the whole string twice, but that seems like the only solution that seems feasible to me.

It is OK to overallocate here.

[rv32ima]

Unified all of the fail cases into one, added more test-cases (including ones where an underscore is at the beginning)