Retooled pushback_str_iterator to make it simpler and faster.
I measured an overall performance improvement of over 22%
when loading a large-ish Perky payload:

    size   164634 bytes
    0.5.5: 0.0806651619495824s
    0.6:   0.06284701998811215s

Also cleaned up some accreted cruft in the test suite(s).