Add tests (especially for serialization)

[pipian]

Although the cueify example has done a good job exercising the APIs and doing basic debugging, we need a proper set of unit tests to ensure that all APIs work as expected (especially serialization/deserialization).

[pipian]

Implementation ongoing. TOC and Multisession APIs now tested; full TOC, MCN/ISRC, CD-Text, Indices still needed.

[pipian]

Almost done. Would like to add one last test of MCN and Indices (specifically, to cover pregaps, and having an MCN without ISRCs). If it demonstrates that read_data_mode works correctly with a Mode 1 data track, all the better.

[pipian]

Just pushed final pregap (at least, based on the indices) test.