The `Amount` and `SignedAmount` were not supposed to implement `serde`
traits by design because doing so implicitly uses sats. We provide two
modules `as_sat` and `as_btc` to allow users to explicitly serialize in
their preferred format.
In commit: `d57ec019d5 Use Amount type for TxOut value field` derives
were added for `serde` and we did not notice it during review.
77085a1fa1