refactor/rewrite concurrent.rs event loop

[dicej]

The event loop and scheduler in concurrent.rs have undergone a lot of changes over time, reflecting significant changes in the component model spec. Some features, such as yielding, backpressure, and waitable sets, were retrofitted onto the existing code in a somewhat haphazard way, making the code more convoluted than it needed to be. In addition, it made control flow hard to understand and debug, with multiply-nested fibers and no clear division of responsibility among tasks and the fibers they ran on.

This commit rewrites most of that code to make it more cohesive:

• No nested fibers: we always suspend the current fiber before resuming the next one
• One single function (poll_until) runs the event loop (rather than several functions with slightly different semantics, as before)
• The code paths for {Typed}Func::call_{async,concurrent} have been unified, reducing redundancy and unsafe code
• We now manage reentrance, yielding, and backpressure by explicitly deferring work as appropriate
  • Work is deferred to either:
    • a high priority queue (work to be done ASAP)
    • a low priority queue (work to be done only once the high priority queue has been drained)
    • a per-instance pending bucket (work that can't be done until a current activation for that instance returns and/or backpressure is unset)
• The new code is more waitable-set-native, removing obsolete ties between parent and child tasks which no longer apply
• Fixed previous unsoundness due to recursively modifying ConcurrentState::futures while polling it
• More types implement Debug, making logs easier to follow