Add strict array operations and perhaps arrays

Currently, `unordered-containers` has a `Data.HashMap.Array` module that's pretty similar to `Data.Primitive.SmallArray`. I'd love to stack `unordered-containers` on top of `primitive` to avoid this duplication. Aside from the bugs currently being fixed in `primitive`, I'm missing some operations. Notably, I need operations like strict `map` (map over an array, forcing the results) and strict `traverse` (traverse an array, forcing the results as they're installed). Could we add such operations?

Furthermore, some strictness bugs that have turned up in `unordered-containers` were caused by failure to force things before installing them in arrays. One possible fix would be to add modules full of strict array operations (the way we have `Data.HashMap.Lazy` and `Data.HashMap.Strict` with the same types). Another option would be to offer a separate type for notionally strict arrays, offering only strict operations (and no `Functor` instance). This would make it easy to avoid goofing up, but users could break open the abstraction if necessary to implement things like efficient `Functor` and `Traversable` instances for types using "strict" arrays.