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

[danburzo]

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