Update language to refer to `u8`s instead of bytes

[joshlf]

As this discussion demonstrates, there is a lot of subtlety around what constitutes a "byte" in Rust's abstract machine. By contrast, it is sufficient to define the bit validity of most types in reference to u8s without actually specifying much about the bit validity of u8 itself. Since most of our conversions operate in terms of existing u8 types (usually [u8]), this isn't a problem for us. We should replace most uses of "byte sequence" with a similar phrase involving u8s.

[RalfJung]

For the curious, we now have a definition of "byte" in the UCG glossary. This is not normative though, it is a WIP definition.