Better doctests for HashMap's into_values and into_keys methods

[ChaiTRex]

The current doctests for a few methods don't really test anything when they could. The following changes would:

• improve the documentation for programmers by better showing what the results of the functions are
• bring the documentation in line with the equivalent methods for BTreeMap, which do include tests like the following
• act as two free tests of those methods

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The current doctest for HashMap's into_keys method can be improved to:

use std::collections::HashMap;

let mut map = HashMap::new();
map.insert("a", 1);
map.insert("b", 2);
map.insert("c", 3);

let mut vec: Vec<&str> = map.into_keys().collect();
vec.sort_unstable();
assert_eq!(vec, ["a", "b", "c"]);

---

The current doctest for HashMap's into_values method can be improved to:

use std::collections::HashMap;

let mut map = HashMap::new();
map.insert("a", 1);
map.insert("b", 2);
map.insert("c", 3);

let mut vec: Vec<i32> = map.into_values().collect();
vec.sort_unstable();
assert_eq!(vec, [1, 2, 3]);

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I would submit a pull request, but my computer is broken. Could someone please make these changes?

[ccqpein]

I did it.