Allow systems to check if an entity has a given component

[RiscadoA]

Problem

Right now we need to OptRead and take it boolean value to know if a component is present.
That's disgusting.

Solution

We would add a system argument used to check whether a given entity has a component. Could also support other stuff such as testing whether a component was just added. E.g.:

void mySystem(ComponentChecker checker)
{
    if (checker.has<Position>(myEntity)) /* ... */
}

[RiscadoA]

This came upon while discussing #389. We need to expose a method somewhere to convert entities from index to full Entity handles. Maybe both of these could be solved in one sweep? Both checking if components exist and getting the generation of an entity from its indices are operations which can safely be executed concurrently.

[RiscadoA]

With the new queries this doesn't seem as important. I will remove this from the current milestone, but keep it here, as we might still find this useful.