This is because Haskell defines negate, abs and signum
for Num, but they don't work with our unsigned number
types. (abs would be a noop.) Haskell has Word32 / Word64,
but there negate doesn't make much sense to me.
Implemented neg and abs.
Implemented a type class inheritance check.
Removed Integral from u8 and i8 since it wasn't implemented.
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