OpenPGP verification of downloaded tarball for GHC

Created by: Franciman

Hi, I tried using gpg to make integrity verification of the downloaded tarball for GHC. It seemed useful to me. For now I just made a sketchy implementation, probably it should be an optional feature and more configurable. Also I could use some advice on how to deal with the public key used for signing ghc tarballs, for now I saved it in a file and told gpg that it's trusted (forcing it with the use of --trusted-key option). Let me know what you think about it!