Create an async API of some kind

[tanriol]

The main options are:

• Reuse the libftdi APIs.
  Upside: close to the upstream APIs.
  Downside: weird integration because these APIs want to drive the libusb event loop themselves.

• Reuse the libftdi APIs, but also provide integration with tokio / async-std.
  Upside: close to upstream and more or less native.
  Downside: a significant amount of nontrivial code.

• Reimplement basic read/write and provide integration with tokio / async-std.
  Upside: more or less native.
  Downside: a significant amount of nontrivial code, including some minimal REing of libftdi.

• Use a separate ftdi thread, provide integration based on futures channels.
  Upside: should be pretty easy.
  Downside: extra latency.

• Something else?