common/channel_type: wrapper for generated 'struct channel_type'.

We make it a first-class citizen internally, even though we won't use
it over the wire (at least, non-experimental builds).  This scheme
follows the latest draft, in which features are flagged compulsory.

We also add several helper functions.

Since uses the *even* bits (as per latest spec), not the *odd* bits,
we have some other fixups.

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

tools/generate-wire.py

@@ -239,6 +239,7 @@ class Type(FieldSet):
         'tx_parts',
         'wally_psbt',
         'wally_tx',
+        'channel_type',
     ]
 
     # Some BOLT types are re-typed based on their field name