mix() function, and 2d/3d counterparts

The idea came up in #207 that it would be nice to have `mix*()` helper functions as shortcuts to linear, bilinear, and trilinear interpolation of colors in a particular color space:

* `mix([colorA, colorB])(t)` is equivalent to `interpolate([colorA, colorB])`
* `mix2d(colors)(tx, ty)` for bilinear interpolation
* `mix3d(colors)(tx, ty, tz)` for trilinear interpolation