Optimized HashMap. RawTable exposes a safe interface.

First, a benchmark of the original hashmap implementation on Intel i3.
```
find_existing    ... 54450 ns/iter (+/- 5524)
find_nonexisting ... 52472 ns/iter (+/- 1672)
find_pop_insert  ...   262 ns/iter (+/- 3)
hashmap_as_queue ...   148 ns/iter (+/- 0)
insert           ...   183 ns/iter (+/- 6)
new_drop         ...   200 ns/iter (+/- 52)
new_insert_drop  ...   317 ns/iter (+/- 104)
```

## `bucket_distance` and `pop_internal`

A branchless implementation of probe count calculation. A branch just avoided unsigned underflow. Since the `capacity` is a power of 2, we can ignore underflow and simply return the difference modulo capacity. We can use the fact that 'an index argument >= capacity' is acceptable later on.

`pop_internal` returns `V` instead of `Option<V>`.

No more low hanging fruits...

## iteration over buckets

Let's use external iterators for their greatest advantage in Rust: to avoid bounds checks and indexing. Thus `{Empty,Full}Index` becomes `{Empty,Full}Bucket`.

The growing is done an optimized reinsertion algorithm based on `insert_hashed_ordered`. `HashMap::new()` returns a table with the capacity set to 0. Hashmaps that never had an element inserted won't allocate, as measured by `bench::new_drop`.

Removed two pointers from `RawTable` as they can be recalculated once per iteration rather than every indexing operation. `RawTable` struct is as small as `Vec` with 24 bytes.

Reduced code duplication between `swap` and `insert_hashed_nocheck`. Created a new method `insert_or_replace_with` that accepts a simple closure for this. Moreover, `robin_hood` will most likely get inlined.

```
find_existing    ... 45556 ns/iter (+/- 1510)
find_nonexisting ... 43812 ns/iter (+/- 1212)
find_pop_insert  ...   224 ns/iter (+/- 9)
hashmap_as_queue ...   134 ns/iter (+/- 0)
insert           ...   183 ns/iter (+/- 14)
new_drop         ...   118 ns/iter (+/- 2)
new_insert_drop  ...   241 ns/iter (+/- 29)
```

## safe interface

This is possible thanks to a lot of prior work by @nikomatsakis! Relevant commit: https://github.com/nikomatsakis/rust/commit/2fcb95b50e5a82c63b82241b98074869b50e1796

To eliminate the double-take issue, we must tie a bucket pointer to a reference to the table within a private structure. Only a bucket that holds a unique reference can be updated. `GapThenFull` uses a similar strategy to encapsulate two consecutive buckets and a single reference. Some of HashMap's methods became functions parameterized over mutability.

We could use an uninit reference or drop trickery to replace one zeroing of a bucket's hash per `shift` of `GapThenFull` with a single memory access. The former language feature is not planned and the latter seems inefficient.

```
find_existing    ... 48131 ns/iter (+/- 3469)
find_nonexisting ... 48518 ns/iter (+/- 1785)
find_pop_insert  ...   238 ns/iter (+/- 5)
hashmap_as_queue ...   131 ns/iter (+/- 2)
insert           ...   217 ns/iter (+/- 6)
new_drop         ...   120 ns/iter (+/- 11)
new_insert_drop  ...   238 ns/iter (+/- 25)
```

## split hashmap.rs into separate files

Total number of lines approaches 3000.

## final refactoring

Standalone `robin_hood`.

I realized that `insert_or_update_with` can call `insert_or_replace_with` directly.

Inlining now started happening in microbenchmarks for some reason. Looks like `search_hashed_generic` is quite small:
```
find_existing    ... 51137 ns/iter (+/- 8663)
find_nonexisting ... 17286 ns/iter (+/- 2961)
find_pop_insert  ...   198 ns/iter (+/- 4)
hashmap_as_queue ...   134 ns/iter (+/- 1)
insert           ...   220 ns/iter (+/- 5)
new_drop         ...   121 ns/iter (+/- 11)
new_insert_drop  ...   234 ns/iter (+/- 6)
```

## conclusion
Further optimizations are increasingly difficult, usually they would make the code unreadable or would require new language features.

It's possible to build cache aware HashMap on top of a vector of SoAs, such as `Vec<([u64, ..N], [K, ..N], [V, ..N])>`. Possibility of in-place growing is the only performance advantage of this approach. Unfortunately, [an increase in iteration complexity](https://github.com/pczarn/code/blob/master/rust/vectrivec/mod.rs) surpasses benefits of storage reuse.

Documentation is not finished. Most methods need more comments and tests.

Robin hood hashing scheme was introduced in #12081.

cc @cgaebel
This function looks pretty similar to `calculate_offsets`. Can you try to share as much code as possible? It was pretty subtle for me to get right in the first place, and bugs in this type of code can be pretty catastrophic.