Async methods to get descriptors

[axelkar]

Currently get_descriptor, get_string_descriptor and get_string_descriptor_supported_languages all block. Could async versions of these methods be made?

[kevinmehall]

On macOS and Linux, get_descriptor just calls control_in_blocking and could just as easily use the async control_in, or you could replicate it with control_in externally. On Windows, it calls IOCTL_USB_GET_DESCRIPTOR_FROM_NODE_CONNECTION. Windows DeviceIoControl takes an overlapped parameter that potentially allows it to be used with IOCP asynchronously, but I haven't tested this.

[kevinmehall]

Implemented in #117 -- these now return MaybeFuture and can be used in blocking or async mode. Implementation on Windows just using the blocking threadpool for now.