This has the benefit of being shorter, as well as more reliable (you
will get a link error if we can't print it, not a runtime one!).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
```
make check-source-bolt CHECK_BOLT_PREFIX="--prefix=BOLT-onion-message" BOLTVERSION=guilt/offers
```
Mainly textual, though I neatened the extra fields check for TLVs with
blinding, and implemented the "no other fields" requirement for
non-final onion message hops.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Instead of open coding in connectd/onion_message, we move it to common
with a nice API.
This lets us process the BOLT test vectors.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
