Improved error handling

Modernize and improve error handling

• Merge the Error and ErrorKind enums.
• Move the Error enum to the error module.
• Derive trait implementations using the thiserror crate.
• Make the Error enum non-exhaustive
• Replace the ChunkIdentifier::try_from method by an implementation of the TryFrom crate.

This release also feature-gates the memmap dependency (the memmap feature is enabled by default).