Rollup merge of #143710 - joshtriplett:random-updates, r=joshtriplett

Updates to random number generation APIs

Updates based on discussions about random number generation.

- Add comment on `RandomSource::fill_bytes` about multiple calls, to allow
  efficient implementations for random sources that generate a word at a time.

- Drop the `Random` trait in favor of `Distribution<T>`, which will let people
  make calls like random(1..=6), and which allows for future expansion to
  non-uniform distributions, as well as floating-point. (For now, this is only
  implemented for `RangeFull`, to get the interface in place. Subsequent PRs
  will implement it for other range types.)

Author: samueltardieu

tests/pass/shims/random.rs

@@ -1,5 +1,5 @@
 #![feature(random)]
 
 fn main() {
-    let _x: i32 = std::random::random();
+    let _x: i32 = std::random::random(..);
 }