Preserve order of compound fields

[samlich]

Keeps the order of compound fields as they are declared in the loaded file using indexmap.

Closes #42

[atheriel]

Since this is a breaking change and may have performance (and other) impacts, I would prefer that it be gated by a feature flag for now. For example, we could add a type definition like the following

#[cfg(feature = "preserve_order")]
type Map = IndexMap<String, Value>;
#[cfg(not(feature = "preserve_order"))]
type Map = HashMap<String, Value>;

and use that instead of the existing HashMap. This would also match the behaviour of the serde_json crate.

[samlich]

Sounds good, I've updated it.

I tried the benchmarks and they seem to improve in general, except for simple_player serialize as struct and complex_player serialize as struct. simple_player serialize as struct doesn't even include a map as far as I can tell, but the results are fairly consistent. It's piqued my interest, so I'll look into it a bit more.

[atheriel]

This looks good to me. I wouldn't pay too much mind to the benchmarks, as I'm not sure the black-box aspect is working correctly in them yet. Thanks!