Issue Description:

The current implementation of FixedWidthInteger.init?(_: radix: ) converts the radix to the receiver type. If that type has less than 6 bits of width, not all radix values are supported and a run error happens at the conversion.

Theoretically, any width above zero should work if we go to a bignum implementation. I'm not suggesting we do that, but that workaround indicates we shouldn't return a nil from the initializer in such cases, because that would imply a user error and not a implementation error. I guess a fatal error is more appropriate.

(BTW, my code was testing with a zero-width type, so there may be other problems.)