Allow the hash salt to be set when constructing a map/set

`Hashable` allows for passing a salt to use when hashing using the `hashWithSalt` method. Not using a default salt (which we do now) is good as it makes it harder for attackers of e.g. web servers that use `HashMap`s to store user supplied data to guess the hashes and create collisions. By allowing  clients to provide this salt when creating the map such attacks could be avoided.
