Convert some packages to nix-based

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

runtimes/crystal.nix

@@ -0,0 +1,31 @@
+{pkgs, piston, ...}:
+let
+    pkg = pkgs.crystal;
+in piston.mkRuntime {
+    language = "crystal";
+    version = pkg.version;
+
+    aliases = [
+        "cr"
+    ];
+
+    compile = ''
+    ${pkg}/bin/crystal build "$@" -o out --no-color
+    chmod +x out
+    '';
+
+    run = ''
+    shift
+    ./out "$@"
+    '';
+
+    tests = [
+        (piston.mkTest {
+            files = {
+                "test.cr" = ''
+                    puts("OK")
+                '';
+            };
+        })
+    ];
+}