Additions:
- Added support for `webp` compression (as per GDAL) under the `webp` feature flag (default: off).
- Added `Decoder::image_ifd`, creating an `IfdDecoder` of the current IFD.
- Added `IfdDecoder::directory`, returning the `Directory` underlying it.
- Added `LowerHex` and `UpwerHex` implementations for `IfdPointer`.

Notes:
- A new example `tiff-ls` demonstrates use of the new features.