ByteString interface

[meooow25]

We could have an interface to work with ByteStrings as a sequence of Word8s, or of ASCII chars, or maybe both.
Implementation would be similar to Text because I expect we want to support slicing.

I'm open to adding this should someone ask for it.

[Bodigrim]

Yes please!

[meooow25]

How do you plan to use it?

I haven't given much thought to the interface here, so it would help to consider some use cases.