Add Perlin noise function

[Kevinpgalligan]

This change gives Sketch a Perlin noise API that is basically lightweight wrapper around noisy

(...which was finally added to the quicklisp depository).

• The API is pretty much identical to p5js, see here.
• Each sketch instance has its own collection of noise states, for thread-safety.
• Also comes with documentation and an example.
• Issue where I originally suggested this.

[Gleefre]

Looks cool!