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
- Fix off-by-one error in String reverse
- Correct order of arguments in strSubstr
- Actually use start index of strSubstr
- Reduce memory usage of strSubstr in case of overrunning string end
- Add fastPack/fastUnpack/fastConcat
- Use unsigned chars for character comparisons
- Fix generated C character encodings
