Convert some packages to nix-based

Affected packages:
- crystal
- dart
- dash
- deno
- elixir
- erlang
- gawk

runtimes/erlang.nix

@@ -0,0 +1,28 @@
+{pkgs, piston, ...}:
+let
+    pkg = pkgs.erlang;
+in piston.mkRuntime {
+    language = "erlang";
+    version = pkg.version;
+
+    aliases = [
+        "erl"
+        "escript"
+    ];
+
+    run = ''
+    ${pkg}/bin/escript "$@"
+    '';
+
+    tests = [
+        (piston.mkTest {
+            files = {
+                "test.erl" = ''
+                    
+                    main(_) ->
+                        io:format("OK~n").
+                '';
+            };
+        })
+    ];
+}