Smooth out the differences between `compiler/utils/Pretty.hs` and `libraries/pretty`
GHC has an internal copy of
- refactor GHC's copy to make it as similar as possible to
pretty, without making any real code changes.
- apply the bug fixes that
prettyreceived to GHC's copy, making sure not to pick up any possible new bugs in the process.
According to (1):
"There is one situation where the laws for pretty are ambiguous and leave room for choice. GHC decided one way and pretty the other."
- Find which law that is, and document the differences.
Ideally we could remove GHC's copy altogether, but we're not there yet. GHC's copy uses FastString, which is supposedly needed for performance, whereas pretty uses