(smoothstep f)
  f: a flo
  returns a flo

Smoothly interpolates between 0.0 and 1.0.

When (<= f 0.0), returns 0.0.

When (>= f 1.0), returns 1.0.

Otherwise, returns (- (* 3.0 f f) (* 2.0 f f f)). This is the smoothstep function.