Profile and tune performance

[gwils]

sv has a benchmark suite, but we have not yet tried to tune performance at all.

With a little bit of profiling, there are probably some low-hanging performance fruit to be had. Profiling will also show the areas on which to focus more serious efforts.

While we're thinking about performance, it might also be worth looking for space leaks using this technique http://neilmitchell.blogspot.com.au/2015/09/detecting-space-leaks.html