ARMv7M: add docs, remove exports

Add Doxygen for the exported ARMv7-M interfaces.

Make the non-exported stuff static.  Remove functions and
data which are now observably unused.

Add comment about a small speedup that the run_algorithm()
logic could use.  Shrink a few too-long lines.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>