Add a new header power.hh for static computations of the power functions

There are lots of power implementations all over our code.
This is the first step of some cleanup.  It moves the class Power_m_p
from misc.hh (like who's ever gonna find it there...) to a separate
header power.hh.  Other implementations will be collected there as
well, if appropriate.

With the patch, including misc.hh will issue a cpp warning that people
who include misc.hh only for Power_m_p should now include power.hh
instead.  This is not great, because it also affects people who include
misc.hh for something else.  Bear with this for a while; I couldn't
think of a better solution.

[[Imported from SVN: r7032]]