nixos/modules: Remove all usages of types.string

And replace them with a more appropriate type

Also fix up some minor module problems along the way

nixos/modules/hardware/video/nvidia.nix

@@ -88,7 +88,7 @@ in
     };
 
     hardware.nvidia.optimus_prime.nvidiaBusId = lib.mkOption {
-      type = lib.types.string;
+      type = lib.types.str;
       default = "";
       example = "PCI:1:0:0";
       description = ''
@@ -98,7 +98,7 @@ in
     };
 
     hardware.nvidia.optimus_prime.intelBusId = lib.mkOption {
-      type = lib.types.string;
+      type = lib.types.str;
       default = "";
       example = "PCI:0:2:0";
       description = ''