RFC: remove erlang distribution, alias to gamma

You can file this under bikeshedding, but given that the Erlang distribution is parametrised exactly the same as a Gamma, do we really need it, especially since most of the methods are delegated to the Gamma anyway? Are there any cases where having the shape parameter constrained to be an integer is useful?

Since the only difference is `rand`: is this a better method for the integer case?