Remove uses of .str(), .c_str() and some other methods of SGPath.
Pass SGPath directly where possible, or explicitly convert to the
appropriate 8-bit encoding.
* experimental clean-up / reduction on two of the FG headers:
(I'm going to await feedback on the developers list before doing more of
these, to avoiding going over files multiple times, but in principle it
seems pretty straightforward.)
* final fixes for SG_USING_STD removal
one file per type in a simple binary conglomeration of packets with no headers
or checksumming (this format is intended for local storage only, not to be
transmitted on the fly over a noisy communication pipe.)
