Implement partition_result

[mingyli]

Addresses #510

partition_result behaves as OCaml Base's partition_result does:

use itertools::Itertools;

let successes_and_failures = vec![Ok(1), Err(false), Err(true), Ok(2)];

let (successes, failures): (Vec<_>, Vec<_>) = successes_and_failures
    .into_iter()
    .partition_result();

assert_eq!(successes, [1, 2]);
assert_eq!(failures, [false, true]);

[jswrenn]

Thank you for this!

bors r+

[bors]

Build succeeded:

• bors build finished