Add a way to customize the literal-ness (single/double quotes) of a string

[sunshowers]

Hi everyone (and epage in particular), thanks for all your hard work on toml! Really appreciating all the work y'all have done integrating toml_edit and toml.

For guppy, tried upgrading toml to 0.7 and sadly ran into a deal-breaking issue. Guppy currently produces output that has single quotes in it, e.g. this file. When I migrate to toml 0.7, it becomes double-quoted as seen in https://gist.github.com/sunshowers/0ed4ab2143ea1efa8afb10f1a500e61e#file-gistfile0-txt-L4-L28.

The output format cannot be changed for backwards compatibility reasons -- these files are checked into repositories and one of the promises made by hakari is to not change how checked-in files are recorded.

So it seems like what I would need is a way to change how strings are recorded so that they're presented as single quotes.

I spent some time looking around and it looks like while this is supported internally, there's no API to access this. Value::String is a Formatted<String>, which doesn't seem to be high-enough fidelity. It seems like what we would really need is a StringWithQuotes or similar, which would track both the string as well as the quoting style.

What do you think? Without this, or some other solution, I'll have to be stuck at toml 0.5 forever. This would be a shame :(