async_executors 0.2.0

Implements Spawn, SpawnLocal and SpawnHandle for commonly used executors.
Documentation
# TODO:

extra tests for async-std:

  - possible to deplete threadpool from it's threads by panicking?

- spawn_blocking? This is provided by tokio and async_std, but does not take a future, rather a closure.
  However it still returns a joinhandle that must be awaited. So if we wrap that in our joinhandle type,
  we now have inconsistent behavior, as both frameworks don't provide any way to cancel the closure when
  the joinhandle get's dropped. We could make a BlockingHandle type?

# Wrap up

- CI - fix windows wasm-bindgen-cli
- clean up benchmark repo
- write blogpost

## Consistent behavior:

The anwser to each point here needs to be the same for all supported executors and the ones from the futures library.

### Spawning

  ✔ what happens if the spawned future panics?

    ✔ on all executors except tokio, the executor thread will unwind. Tokio uses catch_unwind and
      the fact that the future panicked can only be observed by awaiting the joinhandle, but
      the Spawn and LocalSpawn traits do not return a JoinHandle, so there is no way to tell.

  ✔ is spawning fallible or infallible?
     We turn everything to fallible in line with the futures executors.

### JoinHandle

  ✔ provide both detach and drop.
  ✔ what happens if the joinhandle get's dropped.
  ✔ what happens if the future panics.

    ✔ remote handle unwinds the thread that is awaiting the handle
    ✔ async_std unwinds both
    ✔ tokio unwinds none.

    I brought tokio in line with remote_handle, but async_std will unwind the executor thread. This inconsistency remains.