Fix bug where string append leaves heap corrupt for compaction.
string_res.cfa/function "append": do the fix string_res.*/"DEBUG...": enable test to check that compation occurred tests/*: add test of the bug case; this test fails without the fix
Tested for no impact on string concatenation performance. For repeated concatentations, the hot path is the "already at end of heap" case. For concatenations spread across many strings, copying them to the end of the heap dominates.
