Decoding quoted keys with new lines

[alanhdu]

TOML will happily deserialize

"\n" = 5

but if you try and serialize it, it will return toml::ser::Error::KeyNewLine.

So AFAICT, this behavior is motivated by the key-newline.toml test, so this might be expected behavior. But for consistency, I feel like either TOML should refuse to deserialize "\n" as a key, or to go ahead and serialize it as a quoted string key.

Found with https://github.com/rust-fuzz/targets

[alexcrichton]

Aha nice find! I think we should have a special mode of serializing strings here, falling back to this style of escaped newlines for string keys (to make this round-trippable)