Alternate colour map storage

[gennyble]

At birth it used a HashMap containing every colour, which does not seem wise as it was very computationally heavy.

Now it uses a flat vec of every possible 24-bit colour, which is fast as ever but pretty ram heavy. There has to be a middle ground somewhere. It would be neat to have a configurable mapping method so we could experiment a bit and give more choice to crate users. Kind of like Argon2 vs. Argon2i