Make utf8 serialization (including escaping) public

[jorgecarleitao]

We had to copy-paste some code from serde_json (jorgecarleitao/arrow2#813). It would be better if serde_json::ser would offer such a functionality in its public API.

Goal: given a &[u8] representing valid utf8, we would like a function that writes it to a writer (e.g. &mut Vec<u8>) taking into account that some bytes will need to be escaped accordingly.

[dtolnay]

I don't fully understand what your crate is doing that it can't just use the existing API of serde_json, but there isn't anything else not already exposed that I would prefer to expose here.

[jorgecarleitao]

I wanted to serialize a &str to json, but that is indeed already supported because &str implements Serialize. I thought that we had to roundtrip over Value, which would require an allocation. Sorry for the noise. :/