Add compose module and functions

[ripytide]

This PR relies on #647 for the Rect type.

I have more functions that fit in this category I'd like to add in the future but I thought I'd start with some basic ones.

I've added three main functions crop(), flip_horizontal() and flip_vertical().

This is part of the wider effort of migrating function from image::imageops image-rs/image#2238

[theotherphil]

This looks good except for the Rect changes, thanks.

None of these functions are “composing” images in any sense I can think of, but I can’t think of a good name off the top of my head. I’m happy to merge this without the Rect change and worry about module names before the next version is released

[ripytide]

I think I chose the name "compose" from some functions from imageops, mainly replace() and overlay() which combine multiple images hence the name compose. Though you're right the mirror and rotate functions don't really fit but I couldn't think of any other names at the time so I though I'd just put them all in the compose module. Feel free to change the module naming at any point.

[ripytide]

Okay I've removed the Rect changes 👍

[theotherphil]

Thanks! It would be nice to have a benchmark for the cropping function to see if the parallelization actually helps, but that isn't a blocker to merging.