Map-like abstract over anonymous structures

This adds `haxe.ds.DynamicStruct<T>` abstract type that wraps `Dynamic<T>` and `Reflect` calls in a type-safe Map-like API.

This is actually what many newcomers ask for when switching from JavaScript and AS3 and it's particularily useful with JSON/JS objects that are intended to be used as string-keyed collection of values.

I personally use an abstract like this (actually https://gist.github.com/nadako/8370493) for a long time now and I've become too lazy to give this link to every new guy. :wink: