This is an alternative to using `fastUnpack` and `fastPack` to work
on lists of characters.
Using this to refactor the lexer and benchmarking the resulting
compiler on building idris2 shows it's 3 to 5s slower than the
current implementation that goes via `List Char`.
This may be backend-dependent so I still push this to contrib,
with the plan of running further benchmarks in the future.
