Add formatting implementations for LowerHex and friends

[nimish]

Since it's already possible to derive one for Binary, it'd make sense to allow for println("{:x}") and the other integer printing formatters. A workaround is to manually convert to the sized uint before printing but that's a little tedious.

[hecatia-elegua]

Feel free to implement.
You could even just extend it for you own use with a custom derive macro ending its name in Bits.
For structs, the LowerHex impl should just be fmt::LowerHex::fmt(&self.value.value, f).
Actually, wouldn't a "normal" byte based derive LowerHex work?