Use Rust async programming model in Oak Runtime

[blaxill]

Using the async model for channels would likely allow an easy fix for the rust version of #288. It may also make future migration to channels that may be connected to remote runtimes easier.

There are currently some issues when using async features with no_std. Hopefully upstream issues will be fixed soon, and missing functionality can be added to our EAL.

Known issues (non-exhaustive)

• rust-lang/rust - Cannot define async fn with no_std