Dynamically typed queries

hecs could be extended with support for executing queries constructed at runtime rather than specified statically in types. This could be useful for e.g. interactively user-specified queries or for running queries in embedded scripting environments.

An implementation could look something like:

impl World {
    fn query_dynamic(&self, q: DynamicQuery) -> DynamicQueryBorrow<'_>;
}

pub enum DynamicQuery {
    Get(TypeId),
    GetMut(TypeId),
    And(Vec<DynamicQuery>),
}

where DynamicQueryBorrow<'_> contains an iterator which yields DynamicQueryItem which has

impl DynamicQueryItem<'_> {
    fn get<T: Component>(&self) -> Option<&T>;
    fn get_mut<T: Component>(&self) -> Option<&mut T>;
}

or perhaps

impl DynamicQueryItem<'_> {
    fn get(&self, ty: TypeId) -> Option<&dyn Any>;
    fn get_mut(&self, ty: TypeId) -> Option<&mut dyn Any>;
}

This is complex enough that I'm not likely to work on it unless there's significant demand, so please leave a comment describing your use case if you're interested!

The issue proposes a new World::query_dynamic entry point with DynamicQuery, DynamicQueryBorrow, and DynamicQueryItem types, but names no files or tests. Start by reviewing the existing query API and resolving the proposed access model; done would require an agreed design and a complete implementation for runtime-constructed queries.