49914c50f4 Remove `allow_unused` flag in serde (Jamil Lambert, PhD)
Pull request description:
In serde `#[allow(unused_variables)]` has been dropped and the unused variables have been "used" when `debug_assertions` is off.
Close#3332
ACKs for top commit:
tcharding:
ACK 49914c50f4
apoelstra:
ACK 49914c50f4 successfully ran local tests
Tree-SHA512: c83703f91abb5ce513f6403375ce3b2cb24d5200272c98f0a5e532fe41dda1aae8a5334ee8868751e156457f1cfe6b4fe08849fb5329bfbb1c12ed18364e0b1c