New rand_core utility fns + Generator trait

[dhardy]

Draft over #1686 to use the Generator trait.

Mostly only chacha.rs and reseeding.rs differ from #1686.

Performance is now in-line with the old block code:

$ taskset -c 4 cargo bench --bench generators thread -- --baseline master
    Finished `bench` profile [optimized] target(s) in 0.03s
     Running benches/generators.rs (/home/dhardy-extra/.cache/cargo-build/release/deps/generators-4ea267bc6b4453ec)
random_bytes/thread     time:   [310.57 ns 310.73 ns 310.90 ns]
                        thrpt:  [3.0675 GiB/s 3.0691 GiB/s 3.0708 GiB/s]
                 change:
                        time:   [-3.5877% -3.4589% -3.3321%] (p = 0.00 < 0.05)
                        thrpt:  [+3.4470% +3.5828% +3.7212%]
                        Performance has improved.

random_u32/thread       time:   [1.2527 ns 1.2531 ns 1.2535 ns]
                        thrpt:  [2.9719 GiB/s 2.9729 GiB/s 2.9739 GiB/s]
                 change:
                        time:   [-1.6952% -1.6523% -1.6072%] (p = 0.00 < 0.05)
                        thrpt:  [+1.6334% +1.6800% +1.7245%]
                        Performance has improved.
Found 339 outliers among 1000 measurements (33.90%)
  150 (15.00%) low severe
  11 (1.10%) low mild
  33 (3.30%) high mild
  145 (14.50%) high severe

random_u64/thread       time:   [2.0978 ns 2.0982 ns 2.0986 ns]
                        thrpt:  [3.5502 GiB/s 3.5509 GiB/s 3.5516 GiB/s]
                 change:
                        time:   [+2.7585% +2.7905% +2.8206%] (p = 0.00 < 0.05)
                        thrpt:  [-2.7432% -2.7148% -2.6844%]
                        Performance has regressed.
Found 196 outliers among 1000 measurements (19.60%)
  68 (6.80%) low severe
  15 (1.50%) low mild
  79 (7.90%) high mild
  34 (3.40%) high severe

[dhardy]

This needed an explicit impl because it needs an additional where clause: R::Result: Debug

[dhardy]

Unfortunately the available le fns require us to fix W = u32. It would be nice if we could impl ReseedingRng for any W: Word but I don't think there are enough block-RNGs over u64 for this to be a blocking issue.