New unboxing approach about fcore HOT 4 OPEN

@tomtau Are you working on that now?

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I've been waiting for major refactoring to be done, so I can start rewriting in a version that will be easier to merge - I guess most changes are done by now?

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@tomtau yes

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@tomtau, your approach looks very good. There is a slight difference when comparing to miniboxing and specialization in Scala: classes have specialized variants, where only one type of the fields appears (not both primitive and object) -- and all access is done through methods, which are overloaded.

Here's (an unrelated) document that happens to explain this: https://docs.google.com/document/d/12pgg8LBkTNaB5BoeKlBrei-Mst8shdGY7ZPfMFuIYzY/edit?usp=sharing -- just search for miniboxing

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