Support configurable serialization

[crisptrutski]

Closes #73

Sorry for the delay @dotboris, we just missed each other last time I passed through.

Just before implementing this, I realised that using the optional xform parameter (eg. (mr/sync-r ref (partial into (sorted-map)))) should also be sufficient. But that does involve a awkward extra allocation path over the data, and doesn't handle nested maps. Handling those would make it even more awkward 😆

Here I've just made sorted-maps the default. But the implementation leaks out, and the clojure.lang.Sorted property comes at a price: keys are now typed.

((into (sorted-map) {:a 1 :b 2}) :a)
 => 1

((into (sorted-map) {:a 1 :b 2}) "a")
 ;; Java.lang.ClassCastException: java.lang.String cannot be cast to clojure.lang.Keyword

(into (sorted-map) {:a 1 'c 2}) => 
 ;; Java.lang.ClassCastException: java.lang.String cannot be cast to clojure.lang.Symbol

This could break quite a bit of old code, so I'm not completely comfortable with this. Perhaps using something like Linked's order-preserving maps would be preferable, with the added bonus of then working with queries and/or custom priorities.

But I'm loath to add another dependency, and also this "preserve insert order" property is perhaps more confusing than helpful when you start manipulating the data. Or perhaps not a real issue - since the default keys are server generated and monotonically increasing..

The cop out would be to just implement #51 and let poor users choose their trade-off.

That burns down the issue queue fastest, so I'm copping out 😄

[crisptrutski]

Have implemented a rough version, you can try it out with:

(matchbox.serialization.plain/set-default!)
(matchbox.serialization.keyword/set-default!) ;; default
(matchbox.serialization.sorted/set-default!)
(matchbox.serialization.stable/set-default!)

The last option requires linked library to be added manually as a project dependency.

Before merging I would want to speed up the dispatch of m/serialize and m/hydrate. To be cross platform and support advanced compilation, I'm guessing that means using some native mutables.

[crisptrutski]

Now also closes #51

[crisptrutski]

Overhead is probably dealt with sufficiently now. Feels a bit overwrought though, will sleep on it all.