bug#41242: Port feature/native-comp to Windows

[Nicolas Bértolo]

> 1- eln files do not retain the orginal function bytecode.

That should be easy to change.

> in any point of the code you can get the symbol-function of a defined
   function and leak it everywhere.

This is why I said the GC should do it. It should be able to find all
references.

> We can't technically "swap" functions definitions, the best we do is just
> redefine them that is set a certain symbol-function (what late load does).

When the GC finds a Lisp_Object that points to a subr we want to unload,
it should replace it with one that points to its bytecode equivalent version.

> Even worst the function you want to remove could be active in the stack!

You are right. I hadn't considered this. It can still be done, but would need to
wait until the function finishes.