Add benchmarks for msprime.mutate

[jeromekelleher]

We should add some benchmarks to the ASV suite (#1165) so we can track the performance of the various mutate functions. These should be a lot faster than simulations, so it should be easier to benchmark.

I guess we'd want to benchmark all the different models on a small, medium and large tree sequence?

[jeromekelleher]

We'd want to have a fully baked mutation model API first though, I guess (#1070)