Whole is off-by-one

[fabianfreyer]

For example:

let whole_u8: Interval<u8> = Interval::whole();
assert_eq!(whole_u8.lower(), u8::MIN); // Ok
assert_eq!(whole_u8.upper(), u8::MAX); // Fails, whole_u8.upper() is 0xfe, not 0xff.

[ptal]

Well, actually it was a design decision, so the width of the interval can be represented by an unsigned integer of the same kind as the bounds, e.g., the number of elements in the interval whole_u8 is 256, which is cannot be represented in a u8. The functions to get the min and max are u8::min_value() and u8::max_value(), implemented by the trait Width in the file ops.rs.