Incorproate PR comments, correct erroneous description in clightning module, add electrs module, add electrs package, add electrs to nix-bitcoin.nix, add electrs to modules/nix-bitcoin.nix

pkgs/electrs.nix

@@ -0,0 +1,44 @@
+let
+  PkgsSource = builtins.fetchGit { url = "https://github.com/nixos/nixpkgs-channels"; ref = "nixos-18.09"; };
+  defaultPkgs = import PkgsSource {overlays = [(import (builtins.fetchTarball https://github.com/mozilla/nixpkgs-mozilla/archive/master.tar.gz)) ]; };
+  defaultRust = defaultPkgs.latest.rustChannels.nightly.rust;
+  defaultCargo = defaultPkgs.latest.rustChannels.nightly.cargo;
+  defaultBuildRustPackage = defaultPkgs.callPackage (import "${PkgsSource}/pkgs/build-support/rust") {
+    rust = {
+      rustc = defaultRust;
+      cargo = defaultCargo;
+    };
+  };
+
+in { pkgs ? defaultPkgs, rust ? defaultRust, buildRustPackage ? defaultBuildRustPackage }:
+pkgs.lib.flip pkgs.callPackage { inherit buildRustPackage; } (
+  { lib, buildRustPackage, fetchFromGitHub, llvmPackages, clang }:
+
+  let
+    version = "0.4.2";
+
+  in buildRustPackage {
+    name = "electrs-${version}";
+
+    src = fetchFromGitHub {
+      owner = "romanz";
+      repo = "electrs";
+      rev = "5f2d4289dcb98ef283725b3d12f8733a7b9e832b";
+      sha256 = "1lqhrcyd8hdaja5k01a2banvjcbxxcwvb2p7zh05984fpzzs02gr";
+    };
+
+    cargoSha256 = "0v0cc62mx728cqfyz3x1bfh2436yiw2hkv58672j2f45cafcgp2h";
+
+    LIBCLANG_PATH = "${llvmPackages.libclang}/lib";
+    buildInputs = [ clang ];
+
+    meta = with lib; {
+      description = "An efficient re-implementation of Electrum Server in Rust";
+      homepage = https://github.com/romanz/electrs;
+      maintainers = with maintainers; [ tailhook ];
+      platforms = platforms.all;
+    };
+  }
+)
+
+