…lt album.

Proper error handling, and moving the conversion between
{ credentials, volume } and file into the cache module

The locations of credentials, volume and audio are now stored
in three separate Optional<PathBuf>s.
Removed the clearing of the cache if an error occurs. This might
be added again later.

This picks up a fix for librespot-tremor when building with Rust 1.48.

* Fix playback in pulseaudio backend

* Add comment regarding safety

- perf:
  * base62 encoding is an order of magnitude faster (~20x);
  * base16/62 enc/dec and from_uri are several times faster (~2-20x);
  * Let FileId::to_base16() reuse the hex encoder (~20x);

- changes:
  * Add to_uri() method;
  * Make from_uri() error handling consistent;
  * Move audio type from string matching to a SpotifyAudioType factory (private);
  * Implent From/Into<&str> for SpotifyAudioType;
  * Add representation sizes as associated constants (private);

- cs/docs:
  * Add rudimentary docs for most public funcs;
  * Add trivial test cases for the codecs;

Moved conversion of SpotifyAudioType from and to &str directly to traits
Used a simpler and more idiomatic syntax in some places.

Refactor SpotifyId

Bump lewton to 0.10

Merge latest changes from dev

…emove_volume

…sation

Add option to choose between track or album normalisation gain

…ache

Cache build dependencies in Github Workflows

…events

Align PLAYER_EVENT naming

Update CI MSRV, remove Travis. Minor tweaks to device discovery.