feat: framework for parallelizable statistic computation

[laggycomputer]

• Introduce the SiteComposable trait, guaranteeing that a GlobalStatistic can be computed from its parts indepedently, concurrently, and potentially out-of-order.
• Use this trait to create generic code capable of parallelizing any such statistic.

[molpopgen]

The type should also explicitly be Send + Sync? For example, some statistic type containing a raw pointer should be allowed to compile.