Document safety expectations, or the lack thereof

I noticed that the instances for `Map`, `Set`, `IntMap`, and `IntSet` are "unsafe" in the sense that they will happily build structures that don't obey the expected invariants if they are presented with improperly ordered input. Is this acceptable behavior (i.e., is `get` only supposed to be used with trusted input)? Or should the invocations of `fromDistinctAscList` be replaced with `fromList`?

Review the Map, Set, IntMap, and IntSet instances and the fromDistinctAscList invocations mentioned in the issue. Determine whether get is intended for trusted input and document the safety expectations, or establish the required behavior if those constructors should be replaced; completion requires a clear project decision.

Written by the indexing model from the issue text.