More precise erased and generic signatures for trait-, static- forwarders

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Followup from #3452, which we resolved in a slightly simplistic way: by tweaking the generic signatures when we detected that the desirable, precise signature would not meet the invariant that "generic signature erases to erased signature"

If mixin would create a bridge method to match the interface method, it would be free to use a more precise signature (erased + generic) for the forwarder method.

This would also let us revisit the way that List[Char] currently is projected to the Java generic signatuer List[Object], rather than List[Character].

Static forwarders also suffer from the same problem. They don't have an interface method to conform to, but if we use a more precise signature we must adapt arguments/return value with primititve- and derived-value class box/unbox operations. Arguably responsibility for this should be removed from the backend and instead moved back somewhere near erasure/mixin.

I tried much of this once before:

https://github.com/retronym/scala/compare/ticket/3452-rebased

But I fell short when I hit problems with overlapping bridges. The root complexity was trying to share the job of bridging between erasure and mixin. Arguably, this was a sign that we ought to run mixin before erasure.

That branch includes the difficult test cases.

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Imported From: https://issues.scala-lang.org/browse/SI-8293?orig=1
Reporter: @retronym
Blocks #6103