untyped refs

[jpeletier]

This PR adds support for obtaining untyped refs in C++, useful to obtain type-erased refs or to obtain refs to runtime-components or components that are explored via reflection.

flecs::ref<T> now inherits from a common flecs::untyped_ref, so there should be less template code. This naming is similar to flecs::component<T> inheriting from flecs::untyped_component.

flecs::entity class also gets two new overloads of get_ref() that allow to get an untyped_ref if no type is specified at compile-time.

The change is completely backwards-compatible.

The following example illustrates how to use this new feature with runtime components:

    flecs::world world;

    // Create a runtime component:
    auto Position = world.component("Position")
        .member(flecs::I32, "x")
        .member(flecs::I32, "y");

    // Create a test entity:
    auto e = world.entity();

    // Add a component instance to the test entity:
    void* pos_ptr = e.ensure(Position);

    // Create a cursor to manipulate the runtime instance 
    // and set some values
    flecs::cursor cur(world, Position, pos_ptr);
    cur.push();
    cur.member("x");
    cur.set_int(10);
    cur.member("y");
    cur.set_int(20);
    cur.pop();

    // obtain a ref:
    flecs::untyped_ref ref = e.get_ref(Position);

    // Create a new cursor to read from the ref and
    // verify the values:
    cur = flecs::cursor(world, Position, ref.get());

    cur.push();
    cur.member("x");
    assert(cur.get_int() == 10);
    cur.member("y");
    assert(cur.get_int() == 20);
    cur.pop();

[SanderMertens]

LGTM, once conflicts are resolved I'll merge it

[jpeletier]

Conflicts resolved 👍

[SanderMertens]

Awesome!