A byte-valued endian-formatted tiff value buffer

[197g]

Previously we introduced ByteOrder as an explicit, public, property of values and now it makes sense to group it together with the Type to create a type-erased buffer which can represent arbitrary endian-adjusted Tiff values: tags::ValueBuffer. This will let us somewhat cleanly express a link between decoder and encoder which was otherwise spotty due to the former handling types as enum-variants (a closed set), and the latter presenting everything in native types (an open set) and an awkward approach to lists in the decoder. See "Construct ValueBuffer from TiffValue" which is reverted since I don't want to introduce that functionality while demonstrating that it would work and having a commit to go off of; and it may be more sensible to instead have a method to fill the buffer from a Decoder instead of that conversion.

As a consequence we give the Encoder a public interface for creating raw directory entries (that may be written to writer or the offset field). That was previously only possible for entries actually added to a directory encoder's directory while confusingly sharing its TiffValue API with the possibility of writing raw values that are actually being compressed. In terms of semantic clarification that is best left for another PR.