Implement `std::iter::Sum` for color types

## What problem does this solve or what need does it fill?

This allows you to use `.sum()` on a list of colors.

It should be a simple change; I may be willing to contribute if I'm confident it's easy enough.

Search the color types and existing std::iter trait implementations in the Rust source, then inspect nearby tests or examples for color arithmetic. Implement Sum consistently for the relevant color types and add or update tests showing that an iterator of colors can be summed successfully.

Written by the indexing model from the issue text.