Attach size to ForeignPtr

In some types of programs the dominant memory consumer is foreign objects whose lifetime is controlled by ForeignPtrs. In these cases the heuristics used by the GC don't reflect the true heap size of the process, meaning that garbage collection doesn't happen as often as it should.

One suggested approach to alleviating this is to allow users to tell the GC about the size of the object represented by a ForeignPtr.

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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