Disallow post-return from calling imports, allow blocking import calls even when callback is used

[lukewagner]

This PR disallows calling imports during post-return which means that sync-to-sync calls never require a fiber (noting that, if post-return can call imports, it can block after returning-to-and-unblocking the synchronous caller, thereby requiring a fiber to continue asynchronous execution in the callee), preserving the intended cost model of synchronous calls.

This implies that post-return will stick around instead of getting deprecated and removed in the future, thereby removing the reason for having the (as yet unimplemented) always-task-return canonopt. Note that, with only a bit more work, a synchronous export that wants to use task.return could always lift async (no callback) and use backpressure.set to achieve serialized execution.

The PR also relaxes a few of the traps that prevent certain blocking calls from being used in conjunction with callback. These traps were intended to avoid the need for fibers in some cases, but based on implementation experience, we think they'll be too strict and prevent various realistic scenarios where an unaware transitive dependency does a little unexpected blocking. Notes and TODOs are added to Async.md explaining this more.

[dicej]

@lukewagner when you get a chance, would you mind adding a WAT test for this? That will help ensure implementations get it right.

[lukewagner]

You bet; good idea: #535