Add support for ordered map representation of objects

[farlepet]

This adds a new feature ordered-map that will use an IndexMap instead of HashMap in order to preserve the order of items in an object. This features is disabled by default.

This still needs work on the documentation side of things, and perhaps any minor cleanup. I just wanted to put it up as a draft to get some feedback on whether this would be accepted at all (I understand if the consensus is that this doesn't belong in this minimal library).

[cherio]

An alternative to having IndexMap is to have an optional custom property in each Object (e.g. specified by the user) in the existing HashMap e.g. "_properties" which is a JSON array of object keys listed in order as they appear. This could be a compromise that 1) may not affect performance much, 2) achieve the goal of having an ordered list of object properties and 3) still maintain zero dependencies on other crates (speaking of keeping it tiny).

Looking at the code, this could be a minimally intrusive change in parse_object.

[farlepet]

Yeah, back when I originally made this change, it was sort of a quick proof-of-concept to make the ordering more deterministic for a project I was working on. I no longer work at that company, so I no longer have any need for this personally (though if something like this was more widely desirable for this crate I still wouldn't mind helping out).