Big cleanup of the NixOS module system

The major changes are:

* The evaluation is now driven by the declared options.  In
  particular, this fixes the long-standing problem with lack of
  laziness of disabled option definitions.  Thus, a configuration like

    config = mkIf false {
      environment.systemPackages = throw "bla";
    };

  will now evaluate without throwing an error.  This also improves
  performance since we're not evaluating unused option definitions.

* The implementation of properties is greatly simplified.

* There is a new type constructor "submodule" that replaces
  "optionSet".  Unlike "optionSet", "submodule" gets its option
  declarations as an argument, making it more like "listOf" and other
  type constructors.  A typical use is:

    foo = mkOption {
      type = type.attrsOf (type.submodule (
        { config, ... }:
        { bar = mkOption { ... };
          xyzzy = mkOption { ... };
        }));
    };

  Existing uses of "optionSet" are automatically mapped to
  "submodule".

* Modules are now checked for unsupported attributes: you get an error
  if a module contains an attribute other than "config", "options" or
  "imports".

* The new implementation is faster and uses much less memory.

nixos/modules/config/shells-environment.nix

@@ -26,11 +26,10 @@ in
       type = types.attrsOf (mkOptionType {
         name = "a string or a list of strings";
         merge = xs:
-          let xs' = evalProperties xs; in
-          if isList (head xs') then concatLists xs'
-          else if builtins.lessThan 1 (length xs') then abort "variable in ‘environment.variables’ has multiple values"
-          else if !builtins.isString (head xs') then abort "variable in ‘environment.variables’ does not have a string value"
-          else head xs';
+          if isList (head xs) then concatLists xs
+          else if builtins.lessThan 1 (length xs) then abort "variable in ‘environment.variables’ has multiple values"
+          else if !builtins.isString (head xs) then abort "variable in ‘environment.variables’ does not have a string value"
+          else head xs;
       });
       apply = mapAttrs (n: v: if isList v then concatStringsSep ":" v else v);
     };