p5: implement more of p5js API

[sbinet]

we should implement more of the p5js API:

• https://p5js.org/reference/

• path

• rotate

• scale

• transform

• push/pop

• bézier

• curve

• frame-rate

• resize-canvas

• vector (just use gonum.org/v1/gonum/spatial/r2.Vec)

• image loading, image display

• keyboard callbacks

math

• noise (https://p5js.org/reference/#/p5/noise)
• noiseDetail (https://p5js.org/reference/#/p5/noiseDetail)
• noiseSeed (https://p5js.org/reference/#/p5/noiseSeed)