[Feature] Support async functions in rust

[ecton]

I am implementing a UI layer in an async framework, and I am unable to pass Stretch around (including using it as a member on the "self" structure) in async code because MeasureFunc is not Send/Sync.

stretch/src/node.rs

Line 15 in 28e0047

Boxed(sys::Box<dyn Fn(Size<Number>) -> Size<f32>>),

I believe that the change may be as simple as changing the type of Boxed's value to sys::Box<dyn Fn(Size<Number>) -> Size<f32>> + Send + Sync)

This, unfortunately, could be a breaking change since this is a publicly exposed enum, so before jumping in to create a PR, I wanted to see what the opinion would be of attempting this change. I'm new enough to understanding how async works that I am not sure what the likelihood that these functions in most codebase wouldn't already be Send + Sync.