add dynamic query support from feather-hecs

[jazzay]

This brings over the dynamic query changes from a hard fork of hecs:
https://github.com/feather-rs/feather-hecs

This allows us to build and query components dynamically at runtime.

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[Ralith]

Thanks, this looks like a good start on #69! There are, however, some soundness issues as-is, and I think we'll want to iterate on the API a bit.

[Ralith]

This is unsound because &[T] may outlive the query (and indeed the entire World). ComponentSlice should have a lifetime parameter bounded by the lifetime of the query, which should be propagated to this return value.

[Ralith]

This is unsound because no dynamic borrow-checking is performed, and the use of &self here and in World::query_dynamic allows for other outstanding (potentially mutable) borrows. Either something analogous to Fetch::borrow is needed to perform dynamic borrow-checking, or a lot of stuff should be switched to &mut self to statically prevent a dynamic query from coexisting with any other component borrow.

[Ralith]

There should be a safe way to get mutable access to the write_types of the query.

[jazzay]

Thanks for the feedback @Ralith! Will dig into it soon.