Implement Intrinsic Sizing

[LPGhatguy]

A lesser-known part of the Flutter layout algorithm is that objects can report their intrinsic size as opposed to just computing their actual size when performing layout.

This would simplify a lot of widgets and open a lot of doors for us, like being able to implement IntrinsicSize.

Here's some interesting code in Flutter about this: https://github.com/flutter/flutter/blob/f1875d570e39de09040c8f79aa13cc56baab8db1/packages/flutter/lib/src/rendering/box.dart#L1449

API Additions

As part of this, we'll add two new methods to Widget:

    /// Tells the intrinsic width of the object, which is its width if the
    /// widget were given unbounded constraints.
    fn intrinsic_width(&self) -> f32 {
        0.0
    }

    /// Tells the intrinsic height of the object, which is its height if the
    /// widget were given unbounded constraints.
    fn intrinsic_height(&self) -> f32 {
        0.0
    }