Support (a subset of) PSD via the zune-psd crate

[Shnatsel]

The zune-psd crate provides a basic PSD decoder, comparable to that of imagemagick. While the format is very complex and support is far from complete, it is useful for certain applications such as telescope data published in PSD because it doesn't fit into JPEG (the dimensions are larger than 65,535).

It would be nice to add support for it to image. #1877 provides a blueprint for integrating zune-* decoders.

[etemesi254]

FYI, there is the psd crate which offers the ability to merge all layers, so for widespread use, that may be better, since it does layer merging, I don't use it because of the dependency on anyhow which blows up my compile time and I am not impressed with their speeds.

Mine matches ffmpeg/stb decoders.