~aleteoryx/lfm_embed

ref: 94ecd2bba5fb5207d7ea241a8b5762bc2e1376f9 lfm_embed/src/cache d---------
Add SPDX info to all files
Move caching to src/cache/; Finalize API parsing fixes

Font and user cache code has been moved to special files, independant from src/config.rs

API parsing changes have been properly tested, and last.fm API JSON is now trace-logged for debugging convenience.