Main changes are:
1. Uses point32 instead of pubkey32.
2. Uses issuer instead of vendor.
3. Uses byte instead of u8.
4. blinded_path num_hops is now a byte, not u16 (we don't use that yet!).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-EXPERIMENTAL: bolt12: `vendor` is deprecated: the field is now called `issuer`.