lib.types.unique: Check inner type deeply

This doesn't change uniq. Why not?

- In NixOS it seems that uniq is only used with
  simple types that are fully checked by t.check.

- It exists for much longer and is used more widely.

- I believe we should deprecate it, because unique was
  already better.

- unique can be a proving ground.

lib/tests/modules/types-unique.nix

@@ -0,0 +1,27 @@
+{ lib, ... }:
+let
+  inherit (lib) mkOption types;
+in
+{
+  options.examples = mkOption {
+    type = types.lazyAttrsOf
+      (types.unique
+        { message = "We require a single definition, because seeing the whole value at once helps us maintain critical invariants of our system."; }
+        (types.attrsOf types.str));
+  };
+  imports = [
+    { examples.merged = { b = "bee"; }; }
+    { examples.override = lib.mkForce { b = "bee"; }; }
+  ];
+  config.examples = {
+    merged = {
+      a = "aye";
+    };
+    override = {
+      a = "aye";
+    };
+    badLazyType = {
+      a = true;
+    };
+  };
+}