switched to arena malloc to fix #472
added stack_alloc::bytes_allocated and test
added bytes allocated test to softmax test
getting stack_alloc to work under Windows
Added method to inspect the size of the stack. Added test for softmax checking that all elements were allocated to the correct stack.
Merge branch 'bugfix/issue-472-softmax-leak' of https://github.com/stan-dev/stan into bugfix/issue-472-softmax-leak
changed name from cur_size to bytes_requested in stack_alloc
adding a test that could be used for speed tests
Merge pull request #474 from stan-dev/bugfix/issue-472-softmax-leak

Fixes #472.