Use async/await

[dsanders11]

Not going to open a PR for this because I haven't tested it beyond compilation, and not sure it's something you'll want.

Switching the code to using async/await instead of manually creating promises can simplify some of the code, reduces indentation, and shaves 90 lines off the source.

See 5902723453e7f7c215a862d838882f5904dde621 for a commit that switches most code over to this pattern.

In particular, I think devicetoUSBDevice simplifies nicely, as does openDevice. It also makes the code from #39 more robust, as Promise.all will short-circuit if one of the promises rejects, moving on to closing the adapter while the other promises are still unresolved, which might cause an error.

Some food for thought.

[thegecko]

Aync/await in typescript created bulky code when this library was initially written, so it was avoided. However I'm keen to switch to this method of doing things in a lot of my libraries. The problem is time and balancing potential bug introductions with new features :)