Added tests for allFrom*.
Now the benchmarks examine orders of magnitude from 1 to 1000.

This was done to see if any of the functions did
better on a small scale than on a large scale; it
turns out that they did not.
Modified msumBalanced to use an array.

This is faster by a factor of roughly three,
though it does introduce a space overhead in the
case where only a part of the array corresponds to
paths that will be followed (i.e., because we went
down a Path and don't care about the other choices
we could have made).  I decided that this overhead
was going to be a big deal in practice because
there are not going to be many cases where one
branch is followed but the other is not followed
later.
Now the allFrom* functions are mostly copies of their msum counterparts.

There is a slight performance win for not first
mapping "return" over the array.
Changed terminology from MonadStacks to MonadPlusForest (plus more).