chore: refactor the `switch` function for extra clarity

• Remove the idle context from the iterator so there's no need for a skipping logic with a mutable bool;
• Check whether switching to the idle context is a valid option after consuming the entire iterator and not finding a valid context to switch to;
• Decrease one indentation level from the code block that does the switch by using the let Some(foo) = bar else { ... }; syntax;
• Replace ContextRef::upgrade by ContextRef::get_lock, which just returns a reference to the wrapped data (&Arc<_>). No need for Option + filter_map;
• update_runnable doesn't need to be unsafe.