Additional test coverage and compatibility tests

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More test coverage, generically, would be great. In particular, corner cases of the (de)serialization process, and more complex 3rd-party macaroon situations.

There are two sources of existing compatibility tests which could easily be added.

The first are the test vector files from the libmacaroons C library. We would need a simple test harness to load these files, attempt verification, and check that the result matches what is recorded in the test vector file.

The second is the set of tests in the macarooncompat project. We could try to integrate in to that repository, or manually look through the tests and port them back to this repo.