add writeFennelScript function, make ifwait use it

pkgs/write-fennel-script/default.nix

@@ -0,0 +1,21 @@
+{
+  runCommand
+, luaSmall
+, runtimeShell
+, lib
+}:
+let lua = luaSmall;
+in name : packages : source :
+  let
+    luapath = builtins.map (f: "${f}/share/lua/${lua.luaversion}/?.lua;") packages;
+    luacpath = builtins.map (f: "${f}/lib/lua/${lua.luaversion}/?.so;") packages;
+    in runCommand name {} ''
+      #!${runtimeShell}
+      (
+      echo "#!${lua}/bin/lua"
+      echo "package.path = ${lib.strings.escapeShellArg luapath} .. package.path"
+      echo "package.cpath = ${lib.strings.escapeShellArg luacpath} .. package.cpath"
+      ${lua.pkgs.fennel}/bin/fennel --compile  ${source}
+      ) > $out
+      chmod a+x $out
+    ''