support async

[nha]

From the README

This relies on single-threadedness for the duration of a span.

I would like to try using this with https://github.com/juxt/yada/

An alternative design would be to have a lower "core" api where the context (tracer, parent span) would have to be passed explicitly, and another namespace (maybe "sweet" as midje does?) that behaves like the current api.

What do you think?

[emilssolmanis]

I'm not familiar with yada, but it seems like you'd need at least some form of async for it either way. Making that support universal, i.e., not tied directly into a single library, is probably better, and I'd be totally fine with going the midje route.

We also don't expect to be using this asynchronously any time soon, however, so whilst I'd gladly take a PR, I don't think anyone at uSwitch has time to work on getting it there in the near future.