* Use sub-crates of future
* Remove unnecessary pin-project
* Removed unused crates and features
* Replace futures channels by tokio channels
* Use serde's "derive" feature flag instead of serde_derive
* Remove default impl for `SessionConfig`
* Move util mod to single file
* Restore privacy of mods
* Move `fn get_credentials` to application
* Remove `extern crate` statements
* 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
