This reverts commit 623c5564.

Insteads of desugaring `?` in the macro, we can have the async block
itself return `Result<(), E>`, and adjust the supporting code so that
`?` just works. The benefit is that this allows `?` operators that are
hidden behind macros.

On versions prior to 1.45, the compiler will error with
`procedural macros cannot be expanded to statements`.

* Refactor Scrub

* Wrap macro in a declarative macro and use $crate

As a downside we get slightly worse error messages as the filename and
line number will point to the macro definition site instead of the call
site.

* Document for await syntax
* Make `README.md` a symlink to `async-stream/README.md`

This now requires 1.45

* Remove redundant reexports: Pin and Option

* Switch CI to stable Rust

* Fix UI tests to match stable Rust

* Update futures-rs to 0.3.0

* Change impl Stream for AsyncStream to use Self::Item

* Fix doc links to point to futures-core instead of futures-core-preview

* Update READMEs to use the same includes as docs from lib.rs

We get FusedStream for free, given the implementation.  May as well
expose this information to avoid unnecessary fused wrappers.

Bring `try_stream! { ...? }` closer to the usual `?` operator by
converting the error type via `From`.

Fixes #11