String bug fixes and new tests.
Enabled white-box visibility (DEBUG_ functions) into the string representation for heap-oriented tests.
string-gc/basicFillCompact
- newly testable, now with the DEBUG_ visibility, but was basically already working
string-gc/fillCompact_withSharedEdits
- new check for bug fixed here, where an append that triggers a compaction left substrings with dangling pointers
to the old text-pad range; fix is how string_res/assign now sequences growth-pushing operations before grabbing
pointers into the ranges of the old-version
string-overwrite
- new broad check a few of whose cases are fixed here; fixes are the adjustments to the case priorities and
edge-case classifications in string_res/assign "adjust all substring string and handle locations" section
