Api changes to support async api

[ianoc]

This is the API boundary core, without an actual impl.

The test updates here cover the reply.rs async version.

Future PR's needed:

1) Make integration tests support multiple impls 2) Add async examples (1 per PR) 2) Add tokio impl, and update examples to use it

While these are inflight building with all flags enabled will see some unused code. But normal build flags should see no unused code warnings.

[wmanley]

The test updates here cover the reply.rs async version.

I've been working on a sans-io Reply type which should help reduce duplication here. I've raised my WIP as #131 to show.

[cberner]

The test updates here cover the reply.rs async version.

I've been working on a sans-io Reply type which should help reduce duplication here. I've raised my WIP as #131 to show.

Awesome, that'll be great to cut down on duplication! I'll take a look at that PR next.

[ianoc]

I updated the API function name as you suggested, seems better i think.

I made the dependencies and module optional gated on a async_api flag, added new checks around it. and removed the duplication

[cberner]

Merged. Thanks!