min and max methods for Length

[hecrj]

This PR introduces min and max methods to the Length type, allowing users to define minimum and maximum constraints for any widget:

row![
    container(sidebar).width(Fill.min(150).max(200)),
    container(content).width(FillPortion(3).min(300)),
]

The implementation involved introducing additional min/max passes to our flex layout logic, as well as changing the sizing inheritance of widgets to account for constraint propagation over each axis. However, while elements may be visited multiple times, layout is only ever called once for each of them; keeping the single-pass layout philosophy of the library intact.

The changes here also introduce a bunch of new layout integration tests, which should freeze the requirements of the flex logic and the layout system to avoid future regressions.

I suspect there may be plenty of edge cases that the current logic does not handle well; so we will likely have to iterate a bunch.