d1c758f5a4 Add fee_rate::serde re-export (Tobin C. Harding)
Pull request description:
When we added the `fee_rate::serde` module we forgot to re-export it. This is needed so downstream can do specify serde attributes on struct fields.
```rust
#[serde(with = "bitcoin::fee_rate::serde::as_sat_per_kwu")]
rate: FeeRate,
```
ACKs for top commit:
Kixunil:
ACK d1c758f5a4
apoelstra:
ACK d1c758f5a472a4a67cf9c7afa9ef9c0d793a2e16; successfully ran local tests
Tree-SHA512: 6e6f7879d8a0dab59d79f0e41dd5f9f791b72dfb5a1583d0c87ec04216c0a9c0e5c4fb328b93f5298af47b56d898f48717b1641f51295314423e6a569b4677fe