039d6dc289
Clarify explicitly that BTree{Map,Set} are ordered. One of the main reasons one would want to use a BTree{Map,Set} rather than a Hash{Map,Set} is because they maintain their keys in sorted order; but this was never explicitly stated in the top-level docs (it was only indirectly alluded to there, and stated explicitly in the docs for `iter`, `values`, etc.) This PR states the ordering guarantee more prominently.