Expand stetch_contrast() to color images also using u8

[ripytide]

Ideally we'd want to use the Enlargeable trait currently private inside image to make this work for all types rather than just u8, but this is still an improvement that should be good enough for image-rs/image#2238.

[ripytide]

It seems we need to bump the minimal rust version now since one of our dependencies, I think bitstream-io now requires v1.79

[theotherphil]

Thanks! What impact does this have on function run time for the greyscale case?

[ripytide]

Looks like no impact:
Before PR:

test contrast::benches::bench_stretch_contrast                                        ... bench:         660.08 ns/iter (+/- 2.24)
test contrast::benches::bench_stretch_contrast_mut                                    ... bench:           0.25 ns/iter (+/- 0.00)

After PR:

test contrast::benches::bench_stretch_contrast                                        ... bench:         658.86 ns/iter (+/- 7.36)
test contrast::benches::bench_stretch_contrast_mut                                    ... bench:           0.25 ns/iter (+/- 0.00)

[theotherphil]

Thanks!

(Aside: it looks like the _mut variant of that benchmark is optimising out the function and so not actually running .)