Bugfix/issue 472 softmax leak

Summary:  Fixes #472, memory leak in `agrad::softmax()` function in `agrad/rev/matrix/softmax.hpp`

Intended Effect:  As summarized, plus new function `memory::stack_alloc::bytes_allocated` with tests to support testing the fix 

Side Effect: tests for softmax are now brittle due to dependency on stack_alloc initial block size and block growth and the particular data structures used in softmax;  I didn't know how to test any other way.

How to Verify:  Run test in old version (fails), run in new version (passes).  Alternatively, run a bunch of loops computing softmax in old version and monitor memory to see leak; run in new version to see it plugged.

* [x] code review
* [x] kick off integration testing