Add an INIFNITY constant to Size.

[nils-mathieu]

This pull request adds the INFINITY constant to Size.

const INFINITY: Size = Size::new(f64::INFINITY, f64::INFINITY);

I found myself writing a lot of infinite sizes manually to represent "no constraint", and thought it would be cleaner to have it as an associated constant instead.

Potential questions

Is there any other constants we might want for Size? INFINITY is the only one I personally needed, but maybe there are others.

[jneem]

Let's wait on other constants until someone actually wants them.