Have into_ruby return a Cow<[u8]>

[reese]

Part of #832

Currently, we build everything into str/String types, which assume that the string contents are valid UTF-8. This will not always be the case -- Prism can output strings of any Ruby-supported encoding, so this assumption is unsafe and causes failures. Instead, we should be operating on sequences of u8. This PR is the first step in that direction, making into_ruby return a Cow<'_, [u8]> instead of a Cow<'_, str>, as well as updates emit_op to take &[u8] as a proof-of-concept. Later PRs will move other callers to use u8s, though I didn't want to have a single big-bang PR, so this is cut down to something reasonable for reviewability.

The bulk of this PR is updating callsites, primarily converting str literals from "" to b"" or for locs changing from loc_to_str(loc) to loc.as_slice().