How to hash any type to an integer?

[Gary Willoughby via Digitalmars-d-learn]

I'm currently implementing a hash map as an exercise and wondered 
if there is a built-in function I could use to hash keys 
effectively? What I'm looking for is a function that hashes any 
variable (of any type) to an integer.

I've been looking at the `getHash` function of the `TypeInfo` 
class but that only seems to return the passed pointer. Any ideas?