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

As [this discussion](https://github.com/rust-lang/reference/pull/1392#discussion_r1294842256) 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 `u8`s 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 `u8`s.