EntityPath::from_str must parse the given string

[teh-cmc]

This is a very dangerous and hard to track footgun:

    let a = EntityPath::from("null_demo/rect/0");
    let b = EntityPath::from(
        rerun::external::re_log_types::parse_entity_path("null_demo/rect/0").unwrap(),
    );
    dbg!(&a);
    dbg!(&b);
    dbg!(a == b);

[examples/api_demo/src/main.rs:158] &a = EntityPath(null_demo/rect/0)
[examples/api_demo/src/main.rs:159] &b = EntityPath(null_demo/rect/0)
[examples/api_demo/src/main.rs:160] a == b = false

It is made even more dangerous by the fact that some API will just take the EntityPath you give them as-is (e.g. send_path_op) while some will re-parse it behind the scene (e.g. log_arrow).
Consequently, the following will never work, even though everything looks like it's in the right place:

    // sim_time = 1
    MsgSender::new("null_demo/rect/0")
        .with_timepoint(sim_time(1 as _))
        .with_component(&[Rect2D::from_xywh(5.0, 5.0, 4.0, 4.0)])?
        .with_component(&[ColorRGBA::from([255, 0, 0, 255])])?
        .with_component(&[Label("Rect1".into())])?
        .send(session)?;
    MsgSender::new("null_demo/rect/1")
        .with_timepoint(sim_time(1 as _))
        .with_component(&[Rect2D::from_xywh(10.0, 5.0, 4.0, 4.0)])?
        .with_component(&[ColorRGBA::from([0, 255, 0, 255])])?
        .with_component(&[Label("Rect2".into())])?
        .send(session)?;

    // sim_time = 2
    log_cleared(session, &sim_time(2 as _), "null_demo/rect/0", false);

    // sim_time = 3
    log_cleared(session, &sim_time(3 as _), "null_demo/rect", true);

The event log looks right, except it just doesn't work:

[teh-cmc]

cc @emilk you might want to have a look at this one