Add Task::blocking helper

[edwloef]

... and replace usages of task::blocking and task::try_blocking.

The semantics of task::blocking and task::try_blocking weren't entirely clear without reading the function bodies themselves, since they provided the blocking closure with an async channel that you can only try_send on. Most uses only send one item, which is fine since the channel has capacity 1, but sending multiple items can lead to encountering a full channel, which you can't do anything useful about in a blocking context. In the one case where that was necessary (devtools comet installation) silently dropping the message in such situations is probably okay though.

Now, when it's just one item, we can just use the return value of that closure. When it's not, the user can move a sender into the closure and batch with the receiver, so they can choose whether to use an unbounded channel, a bounded channel with potential message loss, or a channel supporting both blocking send and async recv (e.g. tokio, smol, ...).