Remove num-rational

[CryZe]

This removes num-rational as a dependency. As it turns out, num-rational, based on which features are active, can very easily end up on the critical path during compilation, meaning that it is the crate that determines how long it takes to compile the image crate. This is because cargo can parallelize compilation of many crates, but crates with a long dependency chains (rather than wide), are the ones that in the end determine the compilation speed. This is one of the learnings of the serde drama, where it has been noticed that allowing serde and serde_derive to compile in parallel reduces its long dependency chain, resulting in faster compile times. This introduces the same kind of optimization here, where for all the following features, num-rational turns out to be the longest dependency chain:

png,jpeg,gif,bmp,ico,pnm,tga,tiff,webp,hdr,dxt,dds,farbfeld,qoi

The critical path is the following:

autocfg → num-traits compile build.rs → num-traits run build.rs → num-traits → num-integer → num-rational → image

As it turns out, the only thing really used in num-rational is Ratio, which can be easily replicated.

This cuts out both num-integer and num-rational cutting compile times by around 0.5 seconds on a debug build.

[CryZe]

The only behavioral difference is that the ratios don't get simplified anymore. I'm not sure if that's important.