Put DiffArray in its own diffarray package
!DiffArray currently has serious performance problems (see #2727 (closed)), and is not currently being actively maintained. However, as part of the corelib
array it gives the impression that it is a "blessed" library.
I propose that we split !DiffArray off into its own
diffarray package. This also has the advantage that it can then be maintained by a separate maintainer, if someone is interested, and it will be decoupled from GHC's major release schedule.
Deadline: 26 July (2 weeks).