feat: make T: /*snip */ impl Component a feature flag

[sanbox-irl]

This is more of a discussion than a full PR -- I think more documentation would be needed to actually merge this.

This PR is about making hecs require a trait to allow a component to be in the ECS.

I explored three different methods of making hecs require a trait to allow a component to be added to the ECS:

1. The first is this PR. It's kind of a hack, but it's relatively clean. The big loss is any library which uses hecs but then re-exports it, where feature unification will make someone unhappy.
2. The second was making a macro which wraps hecs and allows a user to create "MyOwnHecs" by wrapping every method on hecs::World with a required feature. It was very boilerplate-y, but entirely external to hecs. That isn't such a bad thing! That may still be the best approach since it rocks the boat the least.
3. The final approach was adding a bound which took a trait which was impled by a ZST directly into hecs, like this:

pub trait Satisfies<T: ?Sized> {}

pub struct DefaultPolicy;
impl<T: Send + Sync + 'static + ?Sized> Satisfies<T> for DefaultPolicy {}

pub struct World<P = DefaultPolicy>;

impl<P> World<P> {
    pub fn insert_one<T: Component>(&mut self, e: Entity, c: T) -> Result<(), NoSuchEntity>
    where
        P: Satisfies<T>,
    { /* ... */ }
}

This seemed promising at first, but the ergnomics that P = DefaultPolicy seem to offer really fall apart, and simple code which currently compiles fails to compile (notably let world = World::new()). I think this is the wrong approach right now.

I think it's reasonable to point that this is a non-goal of hecs, but at the same time, every mature game that I know of which uses hecs does seem to wrap hecs such that components must satisfy some trait to be added to the ECS.