Remove unused instruction name propagation

No functional change whatsoever.
Restructure code

In anticipation of upcoming change.
Support decode/encoding of unknown glyf hinting instructions

New instructions can be defined in the prep program.  We don't
check that the "unknown" instruction was actually defined.  Just
pass it through.

Fixes ttx with Skia.ttf on the Mac OS X.

Fixes https://github.com/behdad/fonttools/issues/21
Further fixup to cf75a7c5d05b96df6c6cacde3fdc53e8e6cb200d