Further experimentation with the mode context

Experiments with a larger set of contexts and some
clean up to replace magic numbers regarding the
number of contexts.

The starting values and rate of backwards adaption
are still suspect and based on a small set of tests.
Added forwards adjustment of probabilities.

The net result of adding the new context and forward
update is small compared to the old context from the
legacy find_near function.  (down a little on derf but
up by a similar amount for HD)

HOWEVER.... with the new context and forward update
the impact of disabling the reverse update (which may be
necessary in some use cases to facilitate parallel decoding)
is hugely reduced.

For the old context without forward update, the impact of
turning off reverse update (Experiment was with SB off) was
Derf - 0.9, Yt -1.89, ythd -2.75 and sthd -8.35. The impact was
mainly at low data rates.

With the new context and forward update enabled the impact
for all the test sets was no more than 0.5-1% (again most at
the low end).

Change-Id: Ic751b414c8ce7f7f3ebc6f19a741d774d2b4b556