Random: define Sampler from types, not objects

[rfourquet]

Currently, one defines a sampler on a type with Sampler(rng::AbstractRNG, x, r) = .... This changes this to Sampler(RNG::Type{<:AbstractRNG}, x, r).

The reasoning is that an instance of an RNG shouldn't be used in this function, i.e. only the type is enough to create a Sampler object, which can then be used by different rng instances of the same type.

For convenience, a fall-back Sampler(rng::AbstractRNG, x, r) = Sampler(typeof(rng), x, r) is defined. This is similar to what eltype does.

[StefanKarpinski]

I appreciate improvements, but we've really got to start enforcing feature freeze even for stdlibs very soon. How many people/packages is this likely to disrupt? How many more changes like this do you anticipate needing to make?

[rfourquet]

Yes, I'm embarassed to propose again one more late change. It's the feedback I got recently on the Random.gentype proposal that prompted me to come up with this. I don't anticipe any other change for 0.7.

Technically, it's quite a simple patch which doesn't appear risky, and I believe not a single person or package will be disrupted with this, as the Sampler stuff was implemented in 0.7 and never documented till now (although it's possible to find information about it in a couple of places on github).

So I think it's a good idea to get this in now for beta2; I will merge later today if no objection.

[StefanKarpinski]

Ok, that sounds good. I'm relieved that you don't have a pile more of these 😸