Port ergm.allstats() C backend to use khash.

[krivit]

This will reduce code repetition and also mean that the maximum number of distinct statistics no longer needs to be specified.

This API change may affect ergmito.

[mbojan]

CC @gvegayon

[gvegayon]

Interesting, @krivit and @mbojan. I was looking around, and I couldn't find many references on the performance of this hashing library. (there is one here) Have you run any benchmarks to check how much faster this is? Also, I have been playing a lot with Intel's Advisor, and it is a fantastic tool for optimization (I've used it with R packages and C++ code.) ergm is already pretty fast, still, I wonder if you have checked the code using anything like that.

[krivit]

We haven't, and it would be an interesting project. I chose khash based on those benchmarks, but also because it satisfied a number of requirements:

1. plain C implementation
2. arbitrary data types for keys and values
3. incorporated directly into source: no external libraries required

[gvegayon]

This would be a great project, IMHO! I've created an example repo with some outputs from Intel's Advisor. I am happy to collaborate or provide more info/guidance if you are interested.