Objective

One more step towards #92

This PR is a bit of a mixed bag, so let me know if I should split it up.

The objective of this PR is to set us up so that we can start testing the flexbox algorithm.

Context

While trying to cover the compute_constants() in tests, I discovered the code needed a bit refactoring and massaging in order to actually be testable.

Here I realized that the different resolve() methods can be abstracted to a "Resolve" trait, and following the convention of MaybeMath, we now have MaybeResolve and ResolveOrDefault.

These traits encapsulate behaviour where we need to resolve from a potentially context-dependent size / dimension into a context-independent size / dimensions. In other words:

1. A size can be one (Dimension) or two dimensional (Size<Dimension> / Rect<Dimension>)
2. These sizes can be fully defined on their own:
   1. Dimension::Points(n) tells us that this is n long
3. Or sizes can be dependent on the size of its parent
   1. Dimension::Percent(n) tells us that this is n * parent.width long
   2. Hence "context-dependent"
4. In order perform calculation of the node, we need the sizes to be fully independent of any parent context. This is where the resolve come in.

The only real difference between the two is what they output when they are unable to resolve the size

• MaybeResolve will return a Option<f32> with a None as the fallback
• ResolveOrDefault will return a f32 with the value 0.0 as the fallback

Changelog

• Created a new module resolve which houses all code related code
• Created the trait MaybeResolve
• Create the trait ResolveOrDefault
• Dimension.resolve() and Size<Dimension>.resolve() has been replaced by impl MaybeResolve
• Changed the signature of compute_constants() to make it easier to test
• Added #[must_use] on static constructors that was missing them
• Added convenience constructors on Size<Dimension>, Rect<Dimension>
• [BREAKING] Size<f32>.zero() has now become Size::<f32>::ZERO
• [BREAKING] Point<f32>.zero() has now become Point::<f32>::ZERO
• Added a new() constructor to Size<f32>
• Removed Rect<T>.map as not used anymore

Feedback wanted

• Does the traits make sense and make the code cleaner + easier to understand?
• Docs help plz