Flesh out std::rand::distributions a little more

- Adds the `Sample` and `IndependentSample` traits for generating numbers where there are parameters (e.g. a list of elements to draw from, or the mean/variance of a normal distribution). The former takes `&mut self` and the latter takes `&self` (this is the only difference).
- Adds proper `Normal` and `Exp`-onential distributions
- Adds `Range` which generates `[lo, hi)` generically & properly (via a new trait) replacing the incorrect behaviour of `Rng.gen_integer_range` (this has become `Rng.gen_range` for convenience, it's far more efficient to use `Range` itself)
- Move the `Weighted` struct from `std::rand` to `std::rand::distributions` & improve it
- optimisations and docs