Use .reserve() in HashMap and HashSet's extend methods

[bluss]

It was revealed that hashmaps perform very poorly in this example code:

let first_map: HashMap<u64, _> = (0..900000).map(|i| (i, ())).collect();
let second_map: HashMap<u64, _> = (900000..1800000).map(|i| (i, ())).collect();

let mut merged = first_map;
merged.extend(second_map);

The explanation is that the iterators visit the keys in their bucket
order, and if both maps have the same hash function (including key),
that means that during .extend() the early buckets are overpopulated
long before the whole hashmap's fill level increases to the point that it
triggers it to grow its allocation.

Fixes #36579

[rust-highfive]

r? @aturon

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[bluss]

I have not built & benchmarked the difference yet, but I can't see a reason not to grow the hash map (and set) up front. So it remains to see what the effect is on the extend case.

[jfager]

Isn't this going to hurt performance for maps that have high overlap?

[bluss]

@jfager That is a good point

[bluss]

I retract this. Thanks for the review, it makes sense.