Unboxed sums for performance (Discussion)

It would be useful for us to have a general discussion on places where unboxing and similar techniques might have any benefit. For example there is an old issue here: https://github.com/haskell/containers/pull/523 which tries to use them for `alter` from `IntMap`. In general I would like to gather a list of possible places where we think they could be beneficial so I (or others) can investigate the performance implications.

Start by reviewing the linked containers pull request 523 and its proposed use of unboxing for IntMap alter. Identify other candidate locations and investigate their performance implications; done means producing a reasoned list of worthwhile opportunities rather than changing one named file.

Written by the indexing model from the issue text.