Enable HT RC assertions with escape-hatch

Enable HT RC assertions with escape-hatch

HT functions that modify the array now assert that rc=1. As we don't
respect this COW constraint everywhere, either for a good reason or
because fixing it would take more work, we provide an escape hatch
in the form of HT_ALLOW_COW_VIOLATION(ht). If this macro is called
assertions on this ht are disabled. The macro is a no-op in release
mode.