Remove automatic pixel rounding from AspectRatio

[NickEntin]

See #140 for original motivation:

A (much more breaking) alternative that might be worth considering here is removing pixel rounding from AspectRatio. I'd argue we're being a little too magical today with the rounding, since it's not clear whether your resulting size will be wider/taller than the intended aspect ratio with rounding applied. It would be nice to make any applied rounding more explicit, e.g.

aspectRatio.size(forWidth: ...).ceiledToPixel()

or

aspectRatio.size(forWidth: ...).flooredToPixel()