Add nix-bitcoin.lib for utility functions and types

pkgs/lib.nix

@@ -0,0 +1,5 @@
+{ lib }:
+{
+  # An address type that checks that there's no port
+  ipv4Address = lib.types.addCheck lib.types.str (s: builtins.length (builtins.split ":" s) == 1);
+}