Fix reference type confusion and leak in user random engine

[nielsdos]

No description provided.

[TimWolla]

Oh god. Whether or not a reference is returned is not part of the signature checks?

Functional changes LGTM, some nits regarding the test, since I've taken great care to keep them consistent.

[Girgias]

Oh god. Whether or not a reference is returned is not part of the signature checks?

It is, but you cannot make a by-ref return function return by-val: https://3v4l.org/2OmEj
But you can promote a by-val to by-ref return.