[WIP] Query random access APIs

[caelunshun]

Adds random access APIs for Query and extends random access support using a new EntityAccessor type.

• New EntityAccessor and EntityAccessorMut APIs, which can be created using World::get_accessor() or Query::find_accessor(), allow accessing arbitrary components of an entity. This can be useful for passing an entity and its components to a function without needing access to the World.
• Query::find() attempts to find component data within a query space for a given entity.

let (mut position, velocity) = <(Write<Position>, Read<Velocity>)>::query()
    .find(entity, &mut world)
    .expect("entity not within query space");

Resolves #19.

Note that this depends on the merging of #27.

[caelunshun]

Having a bit of difficulty determining whether an entity matches the query. I see View has associated filter types which have an is_match function, but I'm not sure how to pass the entity to these functions. Any tips?

@TomGillen

[caelunshun]

There is now a random access API in Legion through the SubWorld API, although I feel the EntityAccessor feature in this PR could be of some use. Also, the alternative random access design in this PR—accessing components through queries rather than a single SubWorld—is a solution to #67. Any thoughts on this?

[caelunshun]

I'll close this as #115 provides the EntityAccessor API in the form of World::entity() and World::entity_mut().