Port over osa's original unpack_sums PR

This is an attempt to resuscitate some code from @osa1 from this repo and rebase it on top of current master. It's an attempt at implementing {-# UNPACK #-} for sum-typed fields, as discussed in this GHC wiki article. It's definitely not quite correct as it stands, three ghci-debugger related tests are panicking, and I'm sure that what we did with the Scaled type information is almost certainly wrong as well, though I have no idea how to trigger any infelicity with that.

However, the tests of the feature's actual usage seem like they're working.

Some help with figuring out how to do this correctly would be appreciated!

#15358 (closed) seems like a request for this.

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