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Avoid UArray when indexing is not required

Hannes Siebenhandl requested to merge fendor/ghc:wip/fendor/unlinked-bco-bytearray into master

UnlinkedBCO's can occur many times in the heap. Each UnlinkedBCO references two UArray's but never indexes them. They are only needed to encode the elements into a ByteArray#. The three words for the lower bound, upper bound and number of elements are essentially unused, thus we replace UArray with a wrapper around ByteArray#. This saves us up to three words for each UnlinkedBCO.

Further, to avoid re-allocating these words for ResolvedBCO, we repeat the procedure for ResolvedBCO and add custom Binary and Show instances.

For example, agda's repl session has around 360_000 UnlinkedBCO's, so avoiding these three words is already saving us around 8MB residency.

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