Add isByteArrayCompactable# operation

This feature is motivated by https://www.reddit.com/r/haskell/comments/102ofst/why_cant_bytearray_be_put_in_a_compact_region/ discussion.

ByteArray# can be unpinned or pinned. Unpinned ByteArray# can be compacted, but for pinned arrays as told by isByteArrayPinned# you cannot know.

This is somewhat relevant to ShortByteString type in bytestring. It tells to be pinned, while more correct would be to say "is created with newByteArray# not newPinnedByteArray#", i.e. "compactable". The isByteArrayCompactable# would allow to check the invariant (one can do that by trying to compact the ByteArray# value, but that is indirect and in IO).

I'm not sure whether compactable is the best adjective do describe this property, but I don't know if there are any other than compacting use cases which depend on it.

EDIT: E.g. docs for resizeMutableByteArray# talk about unpinned bytearray#, but I guess they mean "created with newByteArray# as well.

cc bytestring maintainer @Bodigrim.