Co-iteration of sites + their mutations

[molpopgen]

It would be useful to have a treeseq method for this.
There are (at least) two flavors:

1. co-iterate over all sites and the Item of that iterator may generate an iterator over mutations
2. co-iterate over all mutations for a given site.

[molpopgen]

closed via #857, #860, and some earlier ones that beta tested ideas...