Use newly-stabilised `std::ops::ControlFlow`

[lunacookies]

Rust 1.55 stabilised std::ops::ControlFlow, which we defined manually. We should switch to using the definition from the standard library.

[lunacookies]

Our version of ControlFlow has three variants, of which none can be removed. Thus, the only way to start using std’s version is to add a contained boolean to either ControlFlow::Break or ControlFlow::Continue that represents the third variant. This adds complexity and makes the code harder to understand for the only benefit of avoiding one type definition, so I’d argue it’s better to leave the code as it is.