Do not build libsoundio if it is already available on the system.

This is the same as behvaiour as in the origianl `sio_sine.c` example.
The pulseaudio output is still distorted, but it does output a
continuous signal and not a single buffer like before.

`&[&[T]]` was not possible without changing the API. Nonetheless, there
is possibly a more elegant way to pass an array of output frames to the
output method.

`s/@\ _\ //g`

This is zero for PulseAudio.

If the backend is `PulseAudio` then the full buffer size is always used.
This is because `PulseAudio` tries to reduce the buffer size until it
reaches some lower bound. This results in sound artifacts because this
*lower bound* is so low that PulseAudio underflow (/wo getting an
underflow callback) :(

I license past and future contributions to this repository under the
dual MIT/Apache-2.0 license, allowing licensees to chose either at
their
option.

Closes #2

Maybe I should return a `SioResult` in this situation? But, if
the pointer is not null, the conversion function should only fail
if the string content is not valif UTF-8 which is almost never the case
for a name of output stream.

- `inc_ref` is never used (but implementend)
- `InStream` has to be implemented

The name of the multirust update script has changed: `s/blastoff/quick-install/g`