Pre-RFC: add MacAddres to stdlib


#1

Hi all, Diesel recently added support for Postgres’ MACADDR data type, Diesel iteself just uses a [u8; 6] to represent a mac addresses. std::net (https://doc.rust-lang.org/std/net/index.html) would be a great place to add this, as it is used in all sorts of data processing contexts and for plenty utilities.