Fails to build with serde 1.0.168+
https://github.com/monero-rs/monero-rs/blob/main/src/util/amount.rs#L1069-L1088

cautious changed its API in https://github.com/serde-rs/serde/commit/399ef081ecc36d2f165ff1f6debdcbf6a1dc7efb.