Minimal infrastructure for going via TTX rather than SFD

Minimal infrastructure for going via TTX rather than SFD

bedstead.c outputs an utterly minimal TTX file containing no tables, and
TTX compiles it into a similarly minimal OTF.  Which Ghostscript then
complains about.

FontForge is an impressive application, but it's not well-suited to
being part of a mechanical font-production pipeline.  The SFD format is
weird and rather difficult to generate, and FontForge imposes rather
more of its own opinions on the output than I'd like.

TTX is a lower-level format, and while it's a little bit weird, most of
it's weirdness comes from being a faithful representation of the
structure of an OpenType font.  Ideally I'd like something a little
higher-level, but a format that's too low-level is much easier to work
with than one that's too high-level.  The obvious alternative is UFO,
which is very popular but looks like being a pain to generate from plain
C.

Looking to the future, I like the idea of a variable version of
Bedstead, and neither SFD nor UFO seems to support that very well.  In
UFO's case, it seems that the convention is to create UFOs at various
points in the design space and then have a tool interpolate between
them, which seems wrong when they're all procedurally generated from the
same source.  TTX will allow me to directly generate the variation
tables, if I can understand how they work.