Use argument pattern-matching for test_division_rule and remove visibility specifier for test_signed

Having three traits for primitive ints/uints seemed rather excessive. If users wish to specify between them they can simply combine Int with either the Signed and Unsigned traits. For example: fn foo<T: Int + Signed>() { … }

This is a temporary trait until we have default methods. We don't want to encumber all implementors of Ord by requiring them to implement these functions, but at the same time we want to be able to take advantage of the speed of the specific numeric functions (like the `fmin` and `fmax` intrinsics).

These follow the values defined in the C99 standard

The `target_word_size` attribute is always available at compile time, so there is no need for a fallback.