adding LFortran to nixpkgs, call for reviewers

I added LFortran package to NixOS’ packages repository and it’s awaiting review for almost a year now. I kindly ask nix users or anyone interested to review the package (it was my first experience adding a new compiler to nixpkgs), test and comment.

@taranarmo thank you for adding it in! I don’t use NixOS, but if its users could help, that would be awesome.