Cleanup async interface and map 1:1 with Filesystem trait

[mhambre]

This PR takes the experimental async interface and cleans it up such that expansion can start to easily add new methods by mapping 1:1 with the Filesystem trait. This adds a call_fut! macro that sends the task to the tokio runtime, and automatically clones the request and a reference to the runtime to reduce boilerplate.

This is made with the goal in mind of next steps being expansion to include more mutation oriented operations, and orienting the async capabilities in a way that even further down the line this can be backed with AsyncFD, setting up an Async-oriented EventLoop, etc for full end-to-end async.

If this is accepted I can also take the initiative of starting to add the other missing FUSE methods to the async trait and building up a simple_async.rs example FS + tests.

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