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

[tibbe]

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 HashMaps 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.

[sjakobi]

#265 is related.