Generators are still unstable, and generating streams requires complicated
ownership handling, so this crate aims to enable ad-hoc generation of streams.

Current async implementations attempt to provide `yield` syntax via macros,
but this is not transparent to IDEs at the time of this library's creation,
so `gen_z` attempts to work in a way that IDEs understand.

```rust
gen_z(|mut z| async move {
  let awaited = bar().await; // Make awaitable calls in the generator
  z.send(awaited).await; // Streams do not support responses; `yield` has output `()`
  // Share yield helper with calls, allowing them to emit outputs while producing values
  let shared_result = baz(&mut z).await;
  z.send(foo + shared_result).await;
  // Return is infallible; if `Result` is needed, wrap calls to comply with TryStream
}) // => Result is a futures::stream::Stream<Item = T>
```

- `Send` is required for `Stream::Item`. 
  - The stream is `Send` if the provided future is also `Send`.
- `Sync` is available if `Stream::Item` and the future are both `Sync`. 
- The generator only requires `FnOnce`, so it may close over mutable references.
- Infinite streams can be generated by looping within the provided future
- Generators are infallible; the given future must return `()`, run forever, or panic.
  - To use `Result<_, _>`, create an inner-future which returns `Result` and
    convert an `Err` return to a type compatible with `Stream::Item`.
    `TryStream` compatibility can be achieved by setting the `Stream::Item`
    type to `Result<T, TErr>`.