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
* Add utility functions to treat All as a heterogeneous container
* Distinguish RefC Int and Bits types
* Change RefC Integers to be arbitrary precision
* Add RefC Bits maths operations
* Make RefC div and mod Euclidean
* Add RefC bit-ops tests
* Add RefC integer comparison tests
* Add RefC IntN support
