move back samplers

The whole business of moving out core samplers to Sampling.jl does not turn out to make things better. Samplers are closely related to distributions, and separating them into two packages just gets in the way ...

Now the samplers are moved back here, which also comes with a refactored API around `Sampleable` (some basic methods are now defined on `Sampleable` level, such as `size`, `length`, `rand`, and `nsamples`).