Add math.atan2

[gordonbondon]

No description provided.

[bioball]

Need to handle all permutations of long and double here:

Suggested change

	}
	}
	@Specialization
	protected double eval(VmTyped self, double x, double y) {
	return StrictMath.atan2(x, y);
	}
	@Specialization
	protected double eval(VmTyped self, double x, long y) {
	return StrictMath.atan2(x, y);
	}

[bioball]

Let's add some more examples that exercise all permutations.

Suggested change

	math.atan2(4, -3)
	math.atan2(4, -3)
	math.atan2(4.5, -3.5)
	math.atan2(4.5, -3)
	math.atan2(4, 3.5)

Also: maybe a test for correspondence with PI, in the facts section:

facts {
  ["atan2"] {
    math.atan2(1, 0) == math.pi / 2
    math.atan2(0, -1) == math.pi
  }
}

[bioball]

Since we're linking to Wikipedia, let's use verbiage that aligns with it.

Also, need a @Since annotation.

Suggested change

	/// Returns the [4-quadrant inverse tangent][1] of [x] and [y].
	///
	/// [1]: https://en.wikipedia.org/wiki/Atan2
	/// Returns the [2-argument arctangent][1] of [x] and [y].
	///
	/// [1]: https://en.wikipedia.org/wiki/Atan2
	@Since { version = "0.28.0" }

It might also be helpful to have one more paragraph to describe this function's purpose. Maybe something like: "The result is 𝜃 (theta) when converting cartesian coordinates 𝑥 and 𝑦 to polar coordinates 𝑟 and 𝜃."