Add to `units::parse` the complete suit of hex unit parsing functions:
- remove prefix
- assert without prefix
- parse with or without prefix
- parse with prefix
- parse without prefix
- parse prefix unchecked
Refactor `bitcoin` to use the exact function we need, removing code
duplication.
This is a breaking change to `units`, it does however keep the current
re-exports from the public, now empty, `bitcoin::error` module.
a5b93cb159