stretch seems to ignore Text measurements

[hecrj]

If we stop the Text widget from filling the width of its parent by default and its parent aligns items using Align::Start (the default), Text just keeps growing horizontally with no limit:

I don't think this issue can be produced with the current API, so this is an improvement instead of a bug for now.

It seems that stretch chooses the wrong bounding box for the Node. Text should shrink and have its own width, while filling horizontal space of its parent when possible. Maybe I am wrong, but I think that is the whole point of measure functions.

Additionally, when I implemented the Text widget, I noticed that stretch was calling the measure function provided in Node::with_measure way too many times. I was able to optimize this by using a RefCell to cache the first measurement, which seems to be the only one that matters. This last assumption could be wrong, but toggling this optimization seems to have no effect on the issue described above.

In any case, I think both things could be related. We should try to create a simple test in the stretch codebase reproducing the problem and open an issue/PR.