Require manual Component implementation

[Ralith]

Alternative to #454. This is a major breaking change for which I am seeking feedback. It is motivated by several user reports of difficult-to-diagnose errors, and general agreement that inserting foreign types as components is rare and not especially important.

[sanbox-irl]

I implemented this in Fields of Mistria's engine and it + about a minute of adding derives to components, and we were done with no issues. It discovered no new bugs.

One minor issue however was components which had a type parameter(s). This isn't novel to this derive macro (clone has the same issue, for example), where:

#[derive(hecs::Component)]
pub struct Foo<T>(T);

only works when T: Component. That's the standard default and is fine, but it did require me to use my brain a bit to manually write a wrapper impl, which was the appropriate case here.

In any manner, it was a very successful test for us!

[Ralith]

Yeah, sometimes folks offer helper attributes to customize generated bounds, but it's so trivial to implement the trait that I think the simplicity of doing so with conventional syntax wins out.