Exceptions:
- Buffers decoding from strings (usually for tests)
- Buffer.concat, if immediately fed into libsignal, which we know
won't rely on the diverging behavior of slice()
- Encoding GroupIdentifier to base64
Everywhere else, Uint8Array was nearly a drop-in replacement. Places
where it wasn't:
- For tests, use assertArrayEquals more, which converts to hex first
(better diffs anyway)
- Use TextEncoder and TextDecoder for going back and forth between
strings and UTF-8 buffers *not* in the tests
