Added a usage tag.  Docker will pick this up as a 'basic' tag.  This can be handled by the end-use system later on.
Added usage tags to each function.  Yay.
Added a special test folder for proofs of decisions made: echo vs printf, true/false vs 0/1, etc.  Fixed a bug in the core_getopts function: when the last item in compact short options (-abc) requires an argument (-abc 'value for c') it moves OPTIND and doesn't reset __SBT_SHORT_OPTIND to 1.  I think I fixed something else too, but it was minor.  Either way, all tests are passing and I'm clear to move on to the next item.
Removed some old stuff in the bin folder.  Added a new special test.
Added the 'last' option for occurrence in IndexOf.
Touched up IndexOf and wrote the initial version of Substring.  Need to write test cases for it.
Wrote tests for Substring.  Updated the change log.  Fixed an issue in string.sh.  Updated the test suite to make performance tests prettier.