math/unsigned: permit _ separators in Uint128 decimal strings

[kimshrier]

Permit '_' separator characters in strings that can be converted to Uint128 values.

This is like the other string -> numeric value conversions for the builtin number types.

I was thinking of doing the same for Uint256 if this change is acceptable for Uint128.

[penguindark]

May be we need to optimize code like if b_ == '_'.bytes()[0] {
using the bytes() to convert a single char seem a little overkill IMHO.

[kimshrier]

I was following the precedent set in the line following my code addition where '0'.bytes()[0] was used. I am more than willing to change my code in order to be both more efficient and more V like.

[kimshrier]

I wasn't sure if it was appropriate to compare a rune with a u8. Looks like it is.

[spytheman]

I wasn't sure if it was appropriate to compare a rune with a u8. Looks like it is.

Yes, it is somewhat not pure (from a language perspective), but it makes code like this more compact and readable. Rune literals boil down to u8 values, when used in comparison to known u8 values.