Add constructors for network types.

[newAM]

This adds constructors for the network types.

Having a const broadcast address is very helpful for building the application layer (e.g. DHCP from issue #16 ).

This does introduce an API change with regards to the names. I chose these names to align with std::net::Ipv4Addr, which may be available in no_std contexts soon (RFC 2832).

I also derived more default traits to be on par with the implementation of Ipv4Addr in smoltcp.

[kellerkindt]

Thanks for the link to RFC 2832.
+1 for the const-ness, I think last time I tried that, const fns were not stable yet.
I've never heard or read the name Eui48Addr before. Could you elaborate a bit on that or attach a link to a doc?

[newAM]

EUI-48 (Extended Unique Identifiers) is a new term that replaces MAC-48 (source). EUI-48 and MAC-48 differ in name only. I believe the name change was due to the fact that these addresses are now used for other IEEE-802 standards, such as bluetooth and zigbee (source) which use the longer EUI-64.

The name is technically correct, though it is probably needless specificity for this library. Do you think I should change the name back to the original? I do not have a strong opinion on this :)

[kellerkindt]

Thanks for the explanation. Although I now understand the reason for the name change, I think I still prefer to just call it MacAddress. I guess as long as this is not a commonly used term (maybe it is and I'm just living under a rock), others will probably stumble over this as I did. Anyway, it would be really neat if you could add a small explanation (as in the first paragraph of your answer) to the struct, explaining why this should be called EUI-48 - to spread the knowledge! :)

[newAM]

Sounds good to me! I changed the name back to MacAddress, with a small note about being an EUI-48 MAC address.

[kellerkindt]

Thank you!