std: move rand.rs to rand/mod.rs.
std: merge rand::{Rng,RngUtil} with default methods.

Also, documentation & general clean-up:
- remove `gen_char_from`: better served by `sample` or `choose`.
- `gen_bytes` generalised to `gen_vec`.
- `gen_int_range`/`gen_uint_range` merged into `gen_integer_range` and
  made to be properly uniformly distributed. Fixes #8644.

Minor adjustments to other functions.
Remove the C(++) ISAAC Rng from the old rt.

This has to leave rust_gen_seed and rng_gen_seed around since they're
used to initialise the std::rand RNGs.
auto merge of #9362 : huonw/rust/rand, r=alexcrichton

First steps on toward a shiny new `std::rand`, this does a variety of relatively clean-up tasks, like:
- moving `std/rand.rs` to `std/rand/mod.rs` in preparation for (e.g.) splitting the large chunk that is the `Isaac` implementation into a separate file later.
- removing the now unused RNG code in the old rt (500 lines gone just like that!)
- merging Rng and RngUtil via default methods
- examples in the documentation for almost all methods.

And other things mentioned in the commit messages.