Add all_items support in Bag

[jjatria]

This patch adds support for all_items in bag builders.

Like with #177, this is the result of reaching for a feature and realising after the fact that it wasn't supported.

There are work-arounds to not having this, like

is \@array, meta {
    prop this => array { all_items $check; etc };
    prop this => bag { ... };
};

but that feels overly verbose, and I'm not clear on the rationale for not supporting this feature.

If there is a reason for this that I'm missing, we might want to add that to the docs and close this instead.

[exodist]

Thanks!

[jjatria]

Thank you! :D