Convert some packages to nix-based

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

runtimes/elixir.nix

@@ -0,0 +1,30 @@
+{pkgs, piston, ...}:
+let
+    pkg = pkgs.elixir;
+in piston.mkRuntime {
+    language = "elixir";
+    version = pkg.version;
+
+    aliases = [
+        "exs"
+    ];
+
+    run = ''
+    export LC_ALL=en_US.UTF-8
+    ${pkg}/bin/elixir "$@"
+    '';
+
+    tests = [
+        (piston.mkTest {
+            files = {
+                "test.exs" = ''
+                    IO.puts("OK")
+                '';
+            };
+        })
+    ];
+
+    limitOverrides = {
+        "max_file_size" = 100000000; # 100MB
+    };
+}