std: RawTable exposes a safe interface for HashMap

Introduced a new growth algorithm.
std: Split hashmap.rs into modules
std: Refine and document HashMap's code

* branchless `bucket.next()`
* robin_hood is a free function
* fixed the resize policy that was off by one
* documented the growth algorithm
* updated documentation after interface changes
* removed old fixmes
std: Fix overflow of HashMap's capacity