Support sizing image views with aspect ratio preservation

[NickEntin]

It's very common to size image views preserving their image's aspect ratio. We currently support this with the AspectRatio utility, but its call semantics are a bit awkward for this use case. In a simplified example, we end up with something like this:

imageView.bounds.size = imageView.image?.size.aspectRatio.size(toFit: bounds, in: self) ?? .zero

We should add methods calculate the sizeThatFits and to sizeToFit an image view. Additionally these should specify whether upscaling is allowed, since in many cases (e.g. when using a raster image) you want to cap the image at its native size.

func sizeThatFitsPreservingAspectRatio(_ sizeToFit: CGSize, allowingUpscaling: Bool) -> CGSize
func sizeToFitPreservingAspectRatio(_ sizeToFit: CGSize, allowingUpscaling: Bool)