Revert "Remove types.functionTo."

This reverts commit 4ff1ab5.

We need this to type options like:
services.xserver.windowManager.xmonad.extraPackages that specify functions that
take an attribute set containing packages / plugins and return a list containing
a selection of the values in this set.

The reason we need a dedicated type for this is to have the correct merge
behaviour. Without the functionTo type merging multiple function option
definitions results in an evaluation error. The functionTo type merges
definitions by returning a new function that applies the functions of all the
definitions to the given input and merges the result.

lib/types.nix

@@ -349,6 +349,16 @@ rec {
       functor = (defaultFunctor name) // { wrapped = elemType; };
     };
 
+    functionTo = elemType: mkOptionType {
+      name = "function that evaluates to a(n) ${elemType.name}";
+      check = isFunction;
+      merge = loc: defs:
+        fnArgs: elemType.merge loc (map (fn: { inherit (fn) file; value = fn.value fnArgs; }) defs);
+      getSubOptions = elemType.getSubOptions;
+      getSubModules = elemType.getSubModules;
+      substSubModules = m: functionTo (elemType.substSubModules m);
+    };
+
     # A submodule (like typed attribute set). See NixOS manual.
     submodule = opts:
       let