Use iter_mut() instead of mut_iter()

615: Implement RFC #1: FIFO spawns r=nikomatsakis a=cuviper

This implements rayon-rs/rfcs#1, adding `spawn_fifo`, `scope_fifo`, and `ScopeFifo`, and deprecating the `breadth_first` flag.

Fixes #590.
Closes #601.

Co-authored-by: Josh Stone <cuviper@gmail.com>

These adaptors consume may many elements before deferring to a base
folder's fullness checks, and so they need to be performed
manually. For the `filter`s, there's no way to do it manually (rayon-rs#632),
so the specialisations just have to be removed. For `fold` and
`find_any` this can be done with a `take_while`.

This extends the octillion tests to confirm this behaviour.

This makes a program like the following slightly slower compared to
the direct `consume_iter` without a check, but it's still faster than
the non-specialized form.

```
extern crate test;
extern crate rayon;
use rayon::prelude::*;

fn main() {
    let count = (0..std::u32::MAX)
        .into_par_iter()
        .map(test::black_box)
        .find_any(|_| test::black_box(false));
    println!("{:?}", count);
}
```

```
$ hyperfine ./find-original ./find-no-check ./find-check
Benchmark rayon-rs#1: ./find-original
  Time (mean ± σ):     627.6 ms ±  25.7 ms    [User: 7.130 s, System: 0.014 s]
  Range (min … max):   588.4 ms … 656.4 ms    10 runs

Benchmark rayon-rs#2: ./find-no-check
  Time (mean ± σ):     481.5 ms ±  10.8 ms    [User: 5.415 s, System: 0.013 s]
  Range (min … max):   468.9 ms … 498.2 ms    10 runs

Benchmark rayon-rs#3: ./find-check
  Time (mean ± σ):     562.3 ms ±  11.8 ms    [User: 6.363 s, System: 0.013 s]
  Range (min … max):   542.5 ms … 578.2 ms    10 runs
```

(find-original = without specialization, find-no-check = custom
`consume_iter` without `take_while`, find-check = this commit)