Support building on stable channel

[theotherphil]

Might be as easy as just sticking the [bench] tests behind a feature flag.

[bvssvni]

Use cfg_attr like this.

[softprops]

+1

[theotherphil]

Thanks Sven. Doug, I'll sort this one out today.

[theotherphil]

Slightly annoying:

nightly rustc requires:

let mut out = ImageBuffer::<ChannelMap<P, S>, Vec<S>>::new(width, height);

stable rustc requires:

let mut out = ImageBuffer::<ChannelMap<P, S>>::new(width, height);

rust-lang/rust#30123

Luckily it's easy to appease both:

let mut out: ImageBuffer<ChannelMap<P, S>, Vec<S>> = ImageBuffer::new(width, height);

[theotherphil]

Can now build (but not test) on stable, and I've added stable and beta to the CI.