Completely rewrite the libspotify callbacks.

Previously every libspotify callback was using macros defined in
callbacks/util.h to procedurally write for loops (and other common
elements) that would loop over spuc_callbacks[], calling the
libcallbacks callbacks if appropriate.

This commit replaces every macro with a C source function that helps the
process of generating relevant callbacks (previously taken care of by
the macros). Additionally, this commit restructures some of the code and
uses better style.