- `EncodingError` and `DecodingError` are now `#[non_exhaustive]`
- Modified several error paths to return a new variant of `EncodingError` instead of boxing them into
  an `io::Error`, several return `Result` types are adjusted accordingly.
- The `Decoded` enum no longer communicates data from decoded sub-blocks such as the repetition count.
  It now only contains the meta information on framing and extension data. This ensures we are yielding
  less often for performance.
- `last_ext` was renamed to `last_extension_sub_block` for clarity.
- The `Decoder` will now collect XMP and ICC metadata subblocks, making them available after decoding.