b0b0cdb46c Improve the public API for Feerate and Weight (yancy)
Pull request description:
Small nit for https://github.com/rust-bitcoin/rust-bitcoin/pull/1627/ to re-export `Weight` and `FeeRate` to shorten the use path.
```
use bitcoin::Weight;
use bitcoin::FeeRate;
````
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Kixunil:
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Tree-SHA512: 81e508c980c4f37e3791d26616459608dd60e5a6255ef28b3a049c1d27281b2853b92474d42f10031254c8c46878adf666eb709826aa4ffde1b4b3542451e080