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

src/libstd/collections/hashmap/bench.rs

@@ -38,7 +38,7 @@ fn new_insert_drop(b : &mut Bencher) {
 }
 
 #[bench]
-fn insert(b: &mut Bencher) {
+fn grow_by_insertion(b: &mut Bencher) {
     use super::HashMap;
 
     let mut m = HashMap::new();