This min-heap implementation is a pretty standard binomial heap.

## Representation in memory

A heap consists of one or more binomial trees, each with a different order `k`
and `2^k` total nodes. This heap can contain `2^64 - 1` elements at most, which
is far more than your memory can contain, but it's still fun to mention.

A tree does not have its own memory structure; a node that's the root of a
binomial tree is simply called the tree.

Each node has the following layout:

```c
typedef struct vieter_heap_node {
    uint64_t key;
    void *data;
    struct vieter_heap_node *largest_order;
    union {
      struct vieter_heap_node *next_tree;
      struct vieter_heap_node *next_largest_order;
    } ptr;
    uint8_t order;
} vieter_heap_node;
```

Each node has a pointer to its child with the largest order (if the node's
order is `0`, this pointer will be NULL). Each non-root node has a pointer to
its sibling with the next-highest order. These pointers allow the children of a
binomial tree to be recombined into a new tree, once their root has been
pop'ed.

Roots point to the binomial tree in the heap with the next largest order.