* [ performance ] a much faster implementation of unpack
* [ fix ] partiality error in Hangman test
* [ test ] add some documentation to the unpack test case
* [ test ] fix expected output of unpack test
While the discussion about how to refactor test framework is not
finished (#1654), make this change: move `rm -rf build` in the
beginning of the test. For these reasons:
* it is useful to inspect the contents of the `build` directory
especially after the test failure
* if build crashes mid-test (e.g. process killed), next run should
not be affected by the `build` directory from the previous run
