Cache volume across restarts (#220)

* create Volume struct for use with Cache

* add "volume" file to Cache

* load cached volume on start, intial overrides cached overrides default

* amend volume_to_mixer function to cache the volume on every change

* pass cache to Spirc and SpircTask so volume_to_mixer has access

* rustfmt changes

* revert volume_to_mixer function and Spirc/SpircTask cache variable

* Volume implements Copy, pass by value instead of reference

* clamp volume to 100 if cached value exceeds limit

* convert Volume to u16 internally, use float and round to convert hex->dec

* convert initial_volume and ConnectConfig.volume to u16 as well

* add cache_volume function to SpircTask

* remove conversion to/from percentage on cached volume

* consolidate device.set_volume, mixer.set_volume, and caching

* streamline intial volume logic

core/src/lib.rs

@@ -52,3 +52,4 @@ pub mod session;
 pub mod spotify_id;
 pub mod util;
 pub mod version;
+pub mod volume;