Allow for associating multiple queries with single system.

[SanderMertens]

Describe the problem you are trying to solve.
In some scenarios it might be useful to iterate multiple queries in a single system. Currently an application can store queries in a component and access those from a system, but this has as downside that the scheduler is not aware of the components read/written in those queries.

Describe the solution you'd like
Add builtin component that allows for storing multiple queries that is interpreted by scheduler.

[pfeodrippe]

First of all, this is a great project, man, thank you for giving us that! I'm working on an small Clojure game (just me, hopefully will OSS it when things are less messy) and it has been awesome to see how fast things get using Flecs (a query of 1200000 of different small entities takes just 26ms, even with the JNR overhead AND with conversion from data to a clojure map AND while appending it to a vector)!

And this multiple queries feature would indeed be great \o But yes, I see that it has lots of (efficient) workarounds, so we are good \o/