Restructure ColouredString via a common trait.

Restructure ColouredString via a common trait.

Now ColouredString and ColouredStringSlice both implement the trait
ColouredStringCommon, and as many methods as possible are provided by
that trait, depending only on the unimplemented trait methods to get
the text and colours and &str.

This reduces the amount of code, and also makes points of use more
flexible, because I can implement the common trait not just for both
the actual types, but also references to them.

The only disappointment is that I wasn't able to also fold the four
identical impls of std::ops::Add into one via a single
doubly-quantified impl. But instead I got to deploy my first
macro_rules!, so that's something.

In the next commits I'll clean up now-unnecessary boilerplate at call
sites.