Added WASAPI exclusive host

[jesnor]

In exclusive mode only one stream can be opened per device. This is different from the ASIO host which does its own mixing of multiple streams using the sample format of the device. I don't think this is a good idea because it can lead to audio artifacts like lack of precision and overflow. So it's better to let the application do the mixing, preferably using f32 or f64 samples which are then converted to the device sample format.

Also removed duplicated code in the WASAPI Device methods for creating input/output streams.

[JerwuQu]

This is great! I have use for this in an application that requires low-latency audio.

Is there anything currently blocking this? (aside from merge conflicts)

[jesnor]

This PR needs to updated and bug fixed. I could create a new PR later if there is interest in adding WASAPI exclusive to cpal.

[JerwuQu]

Given that there are two open issues #156 and #106 and external projects also waiting for this feature (like @Drewol above, and my own), I'd say there is interest.

I tried the latest version of this branch after rebasing on master, and it seems to have worked with output/speaker but not input/microphone (something about invalid stream format despite using a supposedly supported one), but maybe that's related to the bugs you mentioned.

Just tag me if you want help or testing with anything!