Patch from Paolo Capriotti.

This removes the dependency on the utf8-string package (except for ghc<7.4.1).
A few minor encoding/decoding corner cases were also fixed.

Also remove "import Prelude hiding (catch)" for ghc>=7.5, since
Prelude.catch will be removed from base>=4.6.

This prevents warnings about the stdcall calling convention on Win64.
This patch was submitted by Ian Lynagh.

This change is in preparation for using ghc>=7.4's encodings.

That flag was introduced in ghc-6.10.1, so it's OK to require.

This bumps up haskeline's requirements to ghc-6.10.1 or later.

There was only one place in the source that required it.