Create a utf8 bytestring-a-like
There are a number of things we want a utf8 bytestring-a-like for:
- GHC's !FastString to be built on top of
- Text.!PrettyPrint.ptext (this means we can drop GHC's utils/Pretty)
- template haskell, to use in place of packedstring
- possibly to use in the base IO libraries (see also #2811 (closed))
- possibly to use in haskeline (see also #2812 (closed))
and probably more besides. Ideally all of this will be done comfortably in time for 6.12!