Reduce key dictionary memory footprint

Instead of indexing `Key` objects that have multiple names by each
individual name, index by the set of names. This reduces the size of the
default key dictionary by a factor of between 2 and 3, and as result
also reduces reallocations/defragmentation.
Instead of instantiating all module/extension keys by default, append
them to the `maybe_key`-generator list.

kmk/modules/cg_swap.py

@@ -1,3 +1,4 @@
+from kmk.handlers.stock import passthrough as handler_passthrough
 from kmk.keys import KC, ModifierKey, make_key
 from kmk.modules import Module
 
@@ -12,16 +13,26 @@ class CgSwap(Module):
             KC.LGUI: KC.LCTL,
             KC.RGUI: KC.RCTL,
         }
-        make_key(
-            names=('CG_SWAP',),
-        )
-        make_key(
-            names=('CG_NORM',),
-        )
-        make_key(
-            names=('CG_TOGG',),
+        KC._generators.append(self.maybe_make_cgswap_key())
+
+    def maybe_make_cgswap_key(self):
+        keys = (
+            (('CG_SWAP',),),
+            (('CG_NORM',),),
+            (('CG_TOGG',),),
         )
 
+        def closure(candidate):
+            for names in keys:
+                if candidate in names:
+                    return make_key(
+                        names=names,
+                        on_press=handler_passthrough,
+                        on_release=handler_passthrough,
+                    )
+
+        return closure
+
     def during_bootup(self, keyboard):
         return