common/features: use bitmaps internally, have explicit init function.

This is to prepare for dynamic features, including making plugins first
class citizens at setting them.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_misc.py

@@ -1770,10 +1770,10 @@ def test_list_features_only(node_factory):
                 'option_upfront_shutdown_script/odd',
                 'option_gossip_queries/odd',
                 'option_var_onion_optin/odd',
-                'option_payment_secret/odd',
-                'option_basic_mpp/odd',
                 'option_gossip_queries_ex/odd',
                 'option_static_remotekey/odd',
+                'option_payment_secret/odd',
+                'option_basic_mpp/odd',
                 ]
     assert features == expected