Small simplifications for spin-app

[rylev]

Small tweaks to spin-app after some spelunking. The following changes were made:

• Add some docs. Even though these are internal types, it still helps to have some docs about why they're there.
• Unrolled some type aliases. I find type aliases that are not reused generally make things harder to understand.
• Made DynSafeDynamicHostComponent::update_data_any take just a &mut Data instead of a &mut Box<dyn Any + Send>. There doesn't seem to be any need for that more complicated type
• Made DynamicHostComponents pub(crate)

[lann]

There doesn't seem to be any need for that more complicated type

Probably not. There is a lot of "tweak it till it works then never touch it again" going on in there 😬

[lann]

Unrolled some type aliases. I find type aliases that are not reused generally make things harder to understand.

👍 I probably had these just because I was changing them so frequently trying to understand how to fit things together.

[lann]

Thanks!