UTF escape commands in tokenizer for quoted strings

[clord]

Error from burnt sushi tests:

Test: unicode-escape (valid)

Values for key 'answer4' don't match. Expected a value of 'δ' but got '\u03B4'.

To fix, tokenizer has to decode escape sequences. I have some code that does this somewhere, just have to find it.

• [ ] determine when to decode sequences (single quoted? double quoted? identifiers?)
• [ ] decode sequences
• [ ] which sequences? \u? \x? C-style \r, \n, \t, \0?

[pbrisbin]

This may be valuable: https://github.com/cies/htoml/blob/master/src/Text/Toml/Parser.hs#L258

[clord]

Toml's spec specifies the same list: https://github.com/toml-lang/toml#string