Consider renaming ByteString to IsomorphicString

[annevk]

Context: w3ctag/design-principles#454 (comment).

[domenic]

I'm -0 on this. It's not immediately clear from the name what that means, whereas ByteString seems pretty clear: it's strings where each code point/code unit fits into a byte, to be used in contexts like HTTP where strings are always treated as having byte-sized code points/code units.