add shrink prim-op for other array type

This issue arised in the recently PR to bring unified array interface to primitive: https://github.com/haskell/primitive/pull/64 . In that PR andrewthad ask if we want to keep shrinkArr API. My guts feeling says it's possible for other type of array as well since it's just the same closure overwrite thing. So i'm asking for adding such prim-op here.

I suppose we should provide getSizeOfXXX prim-op which consume a state token for other array type as well since the array size is not referential transparent now.

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I don't think it's quite as simple to provide a shrink operation for MutableArray and MutableArrayArray as it is for MutableByteArray. The reason is that in those cases we would need to remove the truncated references from the GC's remembered set. Of course, this ought to be doable, but it is a bit more complexity.

Sorry, i response to the wrong thread.

Although it will be nice if we can immediately cut the unused reference and save next GC time, But i don't think clearing remembered set is a must to be done after resizing a boxed array, a major GC(or GC on generations which contain the remember set) will eventually collected those object isn't it?

I just can't figure out a way to clear the remembered set efficiently, i hope i'm wrong here.

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