Implement a simple SPC file player

[jonas-schievink]

Depends on #6

SPC format info: https://web.archive.org/web/20070202153330/http://www.alpha-ii.com/snesmusic/files/spc_file_format.txt

The spc700 crate should grow support for saving and loading .spc files. Then we can implement a binary that will play them (this needs to include an audio frontend like cpal, so it might be a good idea to put it in a different crate).