Add color helper functions to dang-math.

# Add color helper functions to dang-math.

All in namespace `dang::math::color`.

## Utility

- clamp
- gray
- rainbow
- color constants

## Conversions

There are various representations that should be convertible to each other:

- rgb (byte and floating point)
- hsv (byte and floating point)
- hex (std::uint32_t)
