Convert some packages to nix-based

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

runtimes/deno-javascript.nix

@@ -0,0 +1,27 @@
+{pkgs, piston, ...}:
+let
+    pkg = pkgs.deno;
+in piston.mkRuntime {
+    language = "javascript";
+    version = pkg.version;
+    runtime = "deno";
+
+    aliases = [
+        "js"
+        "deno-js"
+    ];
+
+    run = ''
+    DENO_DIR=$PWD ${pkg}/bin/deno run $@
+    '';
+
+    tests = [
+        (piston.mkTest {
+            files = {
+                "test.js" = ''
+                    console.log("OK");
+                '';
+            };
+        })
+    ];
+}