Implement missing PrimaryKey trait for CanonicalAddr

[FrankBoltzmann]

The cw_storage_plus Map requires PrimaryKey implementation for its keys, which is implemented for Addr but not CanonicalAddr. However, CanonicalAddr are stable and unique and keys should be stored using that type instead of Addr.

[ethanfrey]

Workaround is using addr.as_bytes() for the key.
In fact, you should never use CannonicalAddr as a key, but rather &CanonicalAddr, it is more efficient and does the same work. The only reason we support Addr in addition to &Addr was for backwards compatibility to not randomly break some older contracts.