Late note: We no longer distribute benchmark images (tests/benches) as part
of the crate itself. Please fetch them from upstream.

Fixes:
- The enumeration types in `tags` are now all marked with a representation of
  their underlying TIFF type (e.g. `repr(u16)`) and variants are explicitly
  assigned their corresponding values. That is you may _read_ the raw
  discriminant and interpret that as the value—except for `Unknown` variants.
- The variants `RationalBig` and `SRationalBig` of `decoder::ifd::Value` are
  deprecated as they have no corresponding TIFF type and were not constructed
  during decoding.

Additions:
- Types in `tags` now generally implement `TiffValue` and can be handed to the
  `DirectoryEncoder::write_tag` method. Unlike primitive types they do _not_
  always implement the trait for slices of themselves.
- Added `tags::ValueBuffer`, a byte-based buffer for tags with a runtime value
  type and count.
- Added `IfdDecoder::find_tag_{buf,bytes}` to read tag values into a
  `ValueBuffer` or byte slice respectively without interpreting them
  immediately within the Rust type system.
- Added `DirectoryEncoder::write_{data,tag}_buf` that takes a `ValueBuffer`
  instead of a statically typed argument.
- Added `DirectoryEncoder::write_entry{,_buf,_bytes}` that encode their
  argument into the file and return an `Entry` for later use, without
  immediately adding a tag-entry pair to the directory.
- The `TiffValue` trait is now implemented for several builtin `tags::*` types
  or slices of them as appropriate for their use as values of their intended
  TIFF tag.
- The `TiffValue` trait is now implemented for `[T; N]` for all types that
  implement it for slices.