Auto merge of #38368 - arthurprs:hm-adapt, r=alexcrichton
Auto merge of #38368 - arthurprs:hm-adapt, r=alexcrichton

Adaptive hashmap implementation

All credits to @pczarn who wrote https://github.com/rust-lang/rfcs/pull/1796 and https://github.com/contain-rs/hashmap2/pull/5

 **Background**

Rust std lib hashmap puts a strong emphasis on security, we did some improvements in https://github.com/rust-lang/rust/pull/37470 but in some very specific cases and for non-default hashers it's still vulnerable (see #36481).

This is a simplified version of https://github.com/rust-lang/rfcs/pull/1796 proposal sans switching hashers on the fly and other things that require an RFC process and further decisions. I think this part has great potential by itself.

**Proposal**
This PR adds code checking for extra long probe and shifts lengths (see code comments and https://github.com/rust-lang/rfcs/pull/1796 for details), when those are encountered the hashmap will grow (even if the capacity limit is not reached yet) _greatly_ attenuating the degenerate performance case.

We need a lower bound on the minimum occupancy that may trigger the early resize, otherwise in extreme cases it's possible to turn the CPU attack into a memory attack. The PR code puts that lower bound at half of the max occupancy (defined by ResizePolicy). This reduces the protection (it could potentially be exploited between 0-50% occupancy) but makes it completely safe.

**Drawbacks**

* May interact badly with poor hashers.  Maps using those may not use the desired capacity.
* It adds 2-3 branches to the common insert path, luckily those are highly predictable and there's room to shave some in future patches.
* May complicate exposure of ResizePolicy in the future as the constants are a function of the fill factor.

**Example**

Example code that exploit the exposure of iteration order and weak hasher.

```
const MERGE: usize = 10_000usize;
#[bench]
fn merge_dos(b: &mut Bencher) {
    let first_map: $hashmap<usize, usize, FnvBuilder> = (0..MERGE).map(|i| (i, i)).collect();
    let second_map: $hashmap<usize, usize, FnvBuilder> = (MERGE..MERGE * 2).map(|i| (i, i)).collect();
    b.iter(|| {
        let mut merged = first_map.clone();
        for (&k, &v) in &second_map {
            merged.insert(k, v);
        }
        ::test::black_box(merged);
    });
}
```

_91 is stdlib and _ad is patched (the end capacity in both cases is the same)

```
running 2 tests
test _91::merge_dos              ... bench:  47,311,843 ns/iter (+/- 2,040,302)
test _ad::merge_dos              ... bench:     599,099 ns/iter (+/- 83,270)
```

GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust