A BigDigit can be added to a BigUint - this is one of several
operations being implemented to allow scalar operations on BigInt and
BigUint across the board.
If a `+` is encountered in the middle of parsing a BigUint, this should
generate an `ParseIntError::InvalidDigit`. Since we can't create that
directly, we get it by trying to parse a `u64` from this point, but of
course `+` is a perfectly valid prefix to a `u64`.
Now we include the previous character in the string passed to `u64`, so
it has proper parsing context to understand what's in error.
Fixes#268.
Sam Cappleman-Lynes
Josh Stone
Phaiax
Kent Overstreet