Add ip_mask_to_prefix_checked

[MarkusPettersson98]

This PR adds some new public APIs: ip_mask_to_prefix_checked, ipv4_mask_to_prefix_checked and ipv6_mask_to_prefix_checked.

Like in #203, the rationale for adding new functions that return an Option instead is so that user's may unwrap the result to trigger a compile-time error if anything is wrong.

An ugly wart of const is that the for construct is not yet available, which means that I had to fall back to using a while loop + loop variable for iteration in ipv6_mask_to_prefix_checked.

I understand if this change is a bit too much, as the const-ification now starts to enforce a coding style which feels a bit unidiomatic. Please, think this one through a bit and do come with feedback!

Edit: These APIs will allow us to add a const ditto of IpNetwork::with_netmask if we wish to do that, which is kind of nice 😊