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,
}
```
In preparation for adding `serde` modules to the `fee_rate` module
create a sub directory.
Create a directory and move `fee_rate.rs` to `fee_rate/mod.rs`.
Tobin C. Harding