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>
cb22015b2a