Add ghcid to shell.nix (#1107)
Add ghcid to shell.nix (#1107)

ghcid is an alternative to a full IDE environment. When running, it
incrementally builds futhark every time you make a change.
HIPERFIT/futhark,HIPERFIT/futhark,diku-dk/futhark,diku-dk/futhark,diku-dk/futhark,diku-dk/futhark,diku-dk/futhark,HIPERFIT/futhark