Stabilize `impl From<[(K, V); N]> for HashMap` (and friends)
In addition to allowing HashMap to participate in Into/From conversion, this adds the long-requested ability to use constructor-like syntax for initializing a HashMap:
```rust
let map = HashMap::from([
(1, 2),
(3, 4),
(5, 6)
]);
```
This addition is highly motivated by existing precedence, e.g. it is already possible to similarly construct a Vec from a fixed-size array:
```rust
let vec = Vec::from([1, 2, 3]);
```
...and it is already possible to collect a Vec of tuples into a HashMap (and vice-versa):
```rust
let vec = Vec::from([(1, 2)]);
let map: HashMap<_, _> = vec.into_iter().collect();
let vec: Vec<(_, _)> = map.into_iter().collect();
```
...and of course it is likewise possible to collect a fixed-size array of tuples into a HashMap ([but not vice-versa just yet](https://github.com/rust-lang/rust/issues/81615)):
```rust
let arr = [(1, 2)];
let map: HashMap<_, _> = std::array::IntoIter::new(arr).collect();
```
Therefore this addition seems like a no-brainer.
As for any impl, this would be insta-stable.
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