Thin but safe wrappers for ALSA.

Very much a WIP at this point, and the API might change, but basic playback/recording should work.

Quickstart guide / API design:

• Most functions map 1-to-1 to alsa-lib functions, e g, ctl::CardInfo::get_id() is a wrapper around snd_ctl_card_info_get_id and the alsa-lib documentation can be consulted for additional information.

• Structs are RAII and closed/freed on drop, e g, when a PCM struct is dropped, snd_pcm_close is called.

• To read and write buffers, call the io method. It will return a separate struct which implements std::io::Read and std::io::Write.

• Error handling - most alsa-lib functions can return errors, so the return value from these is a Result.

• Enumeration of cards, devices etc is done through structs implementing Iterator.