VDOM guards, sparse resource nodes and container to content injection

[Tamschi]

This pull request enables and optimises meta state management infrastructure, which effectively means that component instantiation should now be significantly faster, and that an implicitly content-invalidated VDOM cache like the following is now possible:

use asteracea::components::Remember;
use lignin::ThreadSafe;

// asteracea::component! ( Content()() […] );

asteracea::component! {
    RarelyChanged()()

    <*Remember::<ThreadSafe>
        <*Content>
    >
}

Remember is able to re-inject dyn Invalidator with a wrapper notifying itself of changes.
Content then extracts only this wrapper instead of the original instance, so internal changes can become visible despite not setting up explicit invalidation via Remember's .or_unless parameter.

[Tamschi]

Waiting a bit to see how rust-lang/rust-clippy#8484 will be handled (in the least pressuring way possible. I can likely figure out some kind of workaround, even if it would reduce the readability of the generated code a bit).