Adding angleBetween(vec:, basedOn:) to Vector

I remember using this method a lot when writing examples and tutorials,
explaining angles, so I thought I’d add it as a convenience
Adding tests

Do we need more than this?
Adding lerp test
Adding initializer to Rect

init(Point,Size)
Updating XCTAssert to XCTAssertLessThan
Adding tests for Rect

Couldn’t figure out how to test integral, my values were always
slightly off