Change the lint to `warn` in `units/Cargo.toml`.
Allow `missing_errors_doc` in `amount/serde.rs` and `fee_rate/serde.rs`.
Add missing `# Errors` sections to rustdocs where the lint gives a
warning.
Rust macros, while at times useful, are a maintenance nightmare. And
we have been bitten by calling macros from other crates multiple times
in the past.
In a push to just use less macros remove the usage of the
`impl_from_infallible` macro in the bitcoin, units, and internals crates
and just write the code.
The `FeeRate` type wraps a `u64` but the inner value implicitly contains
information about the unit. As such when serializing and deserializing
the unit information is not explicit and if users try to deserialize
with a different unit their code will be silently buggy.
As we do for Amount; add custom serde modules so that users can
serialize in an explicit unit. Furthermore remove the derived impls
forcing users to make the decision. This is as we do for `Amount`.
With this applied one can write
```rust
#[derive(Serialize, Deserialize)]
pub struct Foo {
#[serde(with = "bitcoin_units::fee_rate::serde::as_sat_per_kwu")]
pub fee_rate: FeeRate,
}
```