* Refactor ColorType
Move transparency and palette data into the ColorType
* Fixup tests
* Make more use of helper functions
* Change BitDepth to u8 representation with TryFrom
* Fix clippy lints
* Don't use unstable language features
* Restore documentation on transparency/palette
* Improve performance consistency
Switch from HashMap / HashSet to IndexMap / IndexSet for consistent iteration order of various options and, as a result, more predictable performance.
* Fix verbose message
* No cloning when restoring original data
* Make reductions return a new uncompressed image
Partially fixes#145
* Async reduction evaluator
* Assert
* Faster bit depth check
* Also try 4-bit depth for small-depth images
* Skip test when using miniz
* Ensure palette is trimmed after depth reduction
Fixes#159
* Fudge factor for reductions to prefer better reductions even if gzip estimation says otherwise
* Move reductions to a module. Make copy instead of changing in-place.
* Alpha reductions
* Immutable color reductions
* Immutable interlace reductions
