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/cache.rs

@@ -7,6 +7,7 @@ use std::path::PathBuf;
 
 use authentication::Credentials;
 use spotify_id::FileId;
+use volume::Volume;
 
 #[derive(Clone)]
 pub struct Cache {
@@ -52,6 +53,23 @@ impl Cache {
     }
 }
 
+// cache volume to root/volume
+impl Cache {
+    fn volume_path(&self) -> PathBuf {
+        self.root.join("volume")
+    }
+
+    pub fn volume(&self) -> Option<u16> {
+        let path = self.volume_path();
+        Volume::from_file(path)
+    }
+
+    pub fn save_volume(&self, volume: Volume) {
+        let path = self.volume_path();
+        volume.save_to_file(&path);
+    }
+}
+
 impl Cache {
     fn file_path(&self, file: FileId) -> PathBuf {
         let name = file.to_base16();