Add Validations

[rye]

As part of #26, we should include some kind of implementation for C-VALIDATE in all of the methods that require the NetAddr to be fully masked.

The worst case I can think of is a NetAddr that isn't masked, which doesn't really cause any harm as masking usually happens regardless. But, if somehow memory gets corrupted and a mask changes, the NetAddr's addr might be wrong. (Think: unsafeness through FFI or something ridiculous like that.)

I think it's sufficient to add debug_assert!'s. Could also add a feature for checks which would be on by default.

Worth some thinking.