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.