Test "negative" fails on 32bit systems

[ignatenkobrain]

---- negative stdout ----
	thread 'negative' panicked at 'assertion failed: `(left == right)`
  left: `None`,
 right: `Some(A)`', tests/tests.rs:192:4

[cuviper]

The code only implements from_u64 and from_i64. The rest will be upcast to larger types by the default FromPrimitive methods. So a 32-bit from_usize(!16) will call from_u64(0xffffffef), not sign-extended in any way, so it won't match the negative discriminant.

Either the test should reflect the difference between 32-bit and 64-bit usize, or the implementation needs to change the way it compares values. I guess this is a design choice for the desired behavior.