Add a function

### Add `hashmap_ensure()` function

A common pattern is:

```c
item = hashmap_get(map, &lookup);
if (!item) {
    hashmap_set(map, &new_item);
    item = hashmap_get(map, &lookup);
}
```

Could you implement something like:

```c
void *hashmap_ensure(struct hashmap *map, const void *item);
```

which returns the existing item if present, otherwise inserts it and returns the newly inserted item?
