Use enlargeable types in to_luma functions.

[kaj]

This is an attempt to fix #1214, using the Enlargeable trait to select a suitable type for the calculation in rgb_to_luma and bgr_to_luma.

Integer pixel types will use suitable integer pixel types for the calculation, while floating point pixel types will use floating point types.

This PR also provides the Enlargeable trait for all Primitive types (luckily, Primitive is not implemented for 128 bit numeric types).

I license past and future contributions under the dual MIT/Apache-2.0 license,
allowing licensees to chose either at their option.

[CrackedP0t]

I'll post my benchmark results here too:

Here's a Gist with the code I used

With this benchmark, the old float method takes ~70 ms, and both the int and enlarge methods take ~20 ms, with the enlarge method usually running a few ms faster.

I think this PR is an excellent solution, but I'm still very interested in the maintainers' opinions

[HeroicKatora]

Maintainers opinion: This is a breaking change. There should be a note somewhere so that something similar can be adopted for 0.24, especially since it also improves performance, but I don't think the accuracy fix itself is worth a major version on its own. In particular it would imply that downstream users wouldn't automatically receive the fix which is half of the motivation, right?

[HeroicKatora]

This was accidentally auto-closed because its base branch got promoted to master and then removed. That's quite unfortunate. I'm still aware of this but if you could do a rebase and reopen it against master that would be perfect.