Transform torch-randomkit into torch-distributions

The package torch-randomkit is now split between
* torch-randomkit proper, which only wraps the univariate RNG from the C
  randomkit
* torch-distributions (i.e., this new repo), which has all the rest:
    - univariate PDF, CDF, Quantile
    - multivariate distributions
    - extra distributions that are not in randomkit
    - nice object structure for distributions, including RNG (which
      call randomkit)