Optimisation of unpackCString#

[Neil Mitchell]

Hi

> PROPOSAL 1: Add the following rules to the simplifier:
>
>  > case unpackCString# "" of  ==> case [] of
>  > case unpackCString# "xyz" of ==> case (C# 'x': unpackCString# "yz") of

I've been wanting to have a go at hacking GHC for a while, and this
seems like a good candidate to start with. If there is no obvious
reason this is a bad idea, I'll try and provide a patch and some
benchmarks/measurements in about a month.

Thanks

Neil