Add OR and AND combinators for system run conditions

[alice-i-cecile]

          > I suspected that the only way to make an OR-combinator might be using a custom System impl, and now you confirmed it. So, I think it's out of scope for this PR.

Agreed. I'd like both OR and AND combinators (to follow-up #7559), but that's out of scope here.

Originally posted by @alice-i-cecile in #7579 (comment)

[joseph-gio]

Just to clarify the behavior of these combinators, they should short-circuit, right? A non-short-circuiting AND combinator would be fairly useless, since you could achieve the same effect by just calling .run_if() multiple times.

[alice-i-cecile]

Yeah, short-circuiting is essential. And should have non-surprising behavior (barring interior mutability) because run conditions are read-only.

[joseph-gio]

In that case, I think it would be good to call these and_then/or_else, in order to emphasize the fact that they short circuit.