We have just released the `hex-conservative` crate, we can now use it.
Do the following:
- Depend on `hex-conservative` in `bitcoin` and `hashes`
- Re-export `hex-conservative` as `hex` from both crate roots.
- Remove all the old hex code from `hashes`
- Fix all the import statements (makes up the bulk of the lines changed
in this patch)
2268b44911