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/lib/eval-config.nix

@@ -19,11 +19,9 @@ rec {
   # Merge the option definitions in all modules, forming the full
   # system configuration.  It's not checked for undeclared options.
   systemModule =
-    pkgs.lib.fixMergeModules configComponents extraArgs;
+    pkgs.lib.evalModules configComponents extraArgs;
 
-  optionDefinitions = systemModule.config;
-  optionDeclarations = systemModule.options;
-  inherit (systemModule) options;
+  config = systemModule.config;
 
   # These are the extra arguments passed to every module.  In
   # particular, Nixpkgs is passed through the "pkgs" argument.
@@ -56,16 +54,11 @@ rec {
           # define nixpkgs.config, so it's pointless to evaluate them.
           baseModules = [ ../modules/misc/nixpkgs.nix ];
           pkgs = import ./nixpkgs.nix { system = system_; config = {}; };
-        }).optionDefinitions.nixpkgs;
+        }).config.nixpkgs;
       in
       {
         inherit system;
         inherit (nixpkgsOptions) config;
       });
 
-  # Optionally check wether all config values have corresponding
-  # option declarations.
-  config =
-    assert optionDefinitions.environment.checkConfigurationOptions -> pkgs.lib.checkModule "" systemModule;
-    systemModule.config;
 }