- `Directory` now implements `FromIterator<(Tag, Value)>`.

Changes:
- The decoder now interprets the `ExtraSamples` tag. The sample count must now
  more strict match the expected value with alpha channels only allowing for
  explicitly denoted unassociated or associated alpha. This effects the
  indicated color type when decoding images with additional samples indicated
  as unspecified relation.  Previously, these may have been interpreted as
  alpha by the total sample count (e.g. RgbA if 4 samples under a photometric
  interpretation of RGB).
- The decoder handles planar data, current limited to non-subsampled channels.
  The `Decoder::read_image` method return planes one-after-another depending on
  the size of the buffer that was passed.
- `Decoder::read_image_to_buffer` now takes `&mut DecodingResult` and resizes
  it according to the required layout. Previously, a borrowed `DecodingBuffer`
  was passed which can be replaced by calling `as_bytes_mut` and
  `read_image_bytes`.
- Several methods of tags are now `const`. Note that does not guarantee any
  particular value when calling these methods.

Fixes:
- Fix a bug in the uncompressed encoder that could lead to short writes, i.e.
  data silently dropped when the underlying writer did not accept all data in a
  single write call.
- Encoding YCbCr data now writes the `ChromaSubsampling` tag as `(1, 1)` to
  indicate no subsampling, instead of leaving it at its default of `(2, 2)`.
- The decoder will reject subsampled YCbCr data as there is no upsampling
  routine, except for JPEG compressed images where the JPEG decoders handles
  this. Since the buffer in that case indicates a full-sized plane for all
  color samples any future support for the tag will upsample all planes (at
  least within this major version).

Additions:
- Added support for the `CieLab` color type.
- Added `DecodingResult::resize_to` to create a buffer with a matching sample
  type and dimensions.
- Added `ByteOrder::native` to access the platform's native endianness.
- Added `ByteOrder::convert` to change the byte-order of values in a byte
  buffer, depending on their `Type` as described at runtime.
- Added `DirectoryOffset::new` to encode a directory whose offset is known to
  the caller but that has not been written through the encoder itself.
- Added `Encoder::extra_samples` to encode images with more samples than their
  color's trait implementation would otherwise suggest.
- Added `DirectoryEncoder::extend_from` to encode multiple tag entries from a
  directory whose values were written to the file by means outside the
  encoder's control.
- Added `Decoder::read_coding_unit_bytes` to retrieve data of corresponding
  coordinates from potentially planar data, which is encoded in multiple chunks
  of the file.