Avoid relying on sed.
Improved error messages and removed calls to shell.

I tried to also remove the need for editing generated ml files, but manipulating
MiniML ASTs seems very hard with the current interface of the extractor. In
fact, it is not really an intermediate language, because it makes sense only for
code generated from Coq terms.
Merge branch 'master' of github.com:lemonidas/QuickChick into experiments
merged with Maxime's fix