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.

tests/test_tapdance.py

@@ -9,6 +9,7 @@ from tests.keyboard_test import KeyboardTest
 
 class TestTapDance(unittest.TestCase):
     def setUp(self):
+        KC.clear()
         self.keyboard = KeyboardTest(
             [Layers(), HoldTap(), TapDance()],
             [