We are trying to make error types stable on the way to v1.0
The current `hashes::Error` is a "general" enum error type with a single
variant, better to use a struct and make the error usecase specific.
Improve the `hashes::Error` by doing:
- Make it a struct
- Rename to `FromSliceError`
- Move it to the crate root (remove `error` module)
Includes usage in `bitcoin`.
06afd52a12