Units::Multiset(T)

Generates an UInt64 hash value for this object.

This method must have the property that a == b implies a.hash == b.hash.

The hash value is used along with == by the Hash class to determine if two objects reference the same hash key.

Subclasses must not override this method. Instead, they must define hash(hasher), though usually the macro def_hash can be used to generate this method.