Adjust edge spans according to exclusion mask

[nspope]

It'd be good to be able to "adjust" the span of edges so as to remove missing sites, that are ubiquitous in real data. It's typical for these exclusion masks to contain very many small intervals -- e.g. small indel sized -- that would dramatically increase the number of edges if removed via TreeSequence.delete_intervals. From a runtime perspective I think it'd be much quicker to use an efficient algorithm for intersecting sorted intervals.