Feature/docs benchmarks example

[StatisMike]

All changes were applied while benchmarking, to make sure there were no regressions made during changes.

Additions:

• examples for wfc adds two examples for WFC algorithm. In addition, the AsRef<WaveCell> impl for WaveCellRef, to allow applying the step-by-step collapse example in more readible way.
• benchmarks - addition of benchmarks, both for wfc-image and wfc (base)
• additional documentation - addition of some documentation for the most important members, as well as a brief overview in wfc/lib.rs with pointers on how to implement the algorithm in external crates. There are still possibilities for better docs, but these pointers alone make the implementation easier, so at least partially fixes Better documentation of public members #32
• clippy suggestions - clippy pointed to some things, I've fixed that.

It should be possible to apply every commit independently, so feel free to cherry-pick.