Merge pull request #1272 from lightning-signer/2022-01-sign-invoice-api

Improve KeysInterface::sign_invoice API

diff --cc fuzz/src/chanmon_consistency.rs
Simple merge

diff --cc fuzz/src/full_stack.rs
Simple merge

diff --cc lightning-invoice/src/utils.rs
Simple merge

diff --cc lightning/src/chain/keysinterface.rs
Simple merge

diff --cc lightning/src/ln/channel.rs
Simple merge