Simplification of 2*2^k



I've checked in some changes to handle simplification of things like
2^2*k and 2*3*2^k.  These seem to work and the test suite works just
fine.

I notice, though, that run time has increased a bit.  rtest14 and
rtest_allnummod have increased from 3.75 sec to 4.05 sec on this one
machine running cmucl.  I'm too lazy to do a comparison between the
current code and the version before these simplifications went in
because other things have changed during that period, including more
tests. 

There are still some issues like 2*a/3*2^k is 2*a*2^k/3 instead of
a*2^(k+1)/3.  I don't plan on fixing this issue any time soon.

I hope this works well and doesn't cause problems and I hope no one
minds the slowdown.

Ray