`exrs v1.0.0` lays architectural groundwork

Hi! I'm proud to announce that I declare this libary version 1.0 today.

However, this is not because of full exr support yet. Instead, I came to the conclusion that the remaining missing features should not be added in a hurry. The recent architecture rework is an improvement that enables us to do all the other missing exr features smoothly. That's why this restructuring lifts this crate to version 1.0.

Improvements:

• support auto crop for all image types and with more control
• the beloved closure style for reading rgba images now supports multiple layers
• arbitrary channels now support reading only one layer and skipping the rest
• even simpler "one liner" rgba read and write functions
• reduce code duplication within exrs
• allow you to insert your own modular reader or writer, supporting custom pixel storage types, channels, or even layers
• documentation improvements and lots of improved names