Add dhat?

[kcaffrey]

I added dhat to my repo (which was generated from the template), so that I can run and get output like the following:

$ cargo solve 5 --dhat
    Finished dhat [optimized + debuginfo] target(s) in 0.01s
     Running `target/dhat/05`
dhat: Total:     23,920 bytes in 244 blocks
dhat: At t-gmax: 8,176 bytes in 12 blocks
dhat: At t-end:  0 bytes in 0 blocks
dhat: The data has been saved to dhat-heap.json, and is viewable with dhat/dh_view.html
Part 1: 382895070 (6.2ms)
dhat: Total:     32,528 bytes in 307 blocks
dhat: At t-gmax: 8,384 bytes in 11 blocks
dhat: At t-end:  0 bytes in 0 blocks
dhat: The data has been saved to dhat-heap.json, and is viewable with dhat/dh_view.html
Part 2: 17729182 (4.2ms)

Do you have any interest in getting this upstream? I'm not sure how many people would care to use it. I was mainly curious about optimizing allocations in various computations (e.g. using custom iterators instead of allocating vectors for temporary results) as a learning experience, so I found it useful to be able to see what's going on in the memory allocator.

[fspoettel]

This is cool, I would actually be interested in that myself. If I read it correctly, the whole thing is behind a feature flag? That would be perfect then, and a PR appreciated.