Convert mutable layout getters to layout setters

[nicoburns]

Objective

This makes it easier to integrate with Taffy as:

• It means you don't necessarily have to store the layout as a taffy::Layout struct
• It allows you to run logic just after the layout is set

It also makes it clear that Taffy's algorithms do not read from the layout (and removes the one case where they did!).

Context

This change is motivated by the Xilem integration linebender/xilem#140