ART: Fix a few issues around lower bound / erase (#563)

* ART: Fix a few issues around lower bound / erase

1. Leaf keys were written to the wrong location.
2. In `art_node_iterator_lower_bound`, we did not check if the key chunk for the
   lower bound child was greater than the key chunk we searched for.
3. In `art_iterator_erase` there were a few issues that would lead to an
   incorrect location. I've somewhat sidestepped the most tricky bit by doing a
   lower bound search from the root instead.

Added tests for 1 and 2.

* Improve art_printf

* Makes the representation more compact
* Uses 2-wide hex representation for key chunks

Before:
```
{
 type: Node4
 prefix_size: 2
 prefix: 3030
 key: 30
 {
  type: Node4
  prefix_size: 2
  prefix: 3030
  key: 31
  {
   type: Leaf
   key: 303030303031
  }
  key: 32
  {
   type: Leaf
   key: 303030303032
  }
  key: 33
  {
   type: Leaf
   key: 303030303033
  }
  key: 34
  {
   type: Leaf
   key: 303030303034
  }
 }
 key: 31
 {
  type: Leaf
  key: 303031303035
 }
}
```

After:
```
{
 type: Node4
 prefix_size: 2
 prefix: 3030
 key: 30 {
  type: Node4
  prefix_size: 2
  prefix: 3030
  key: 31 { type: Leaf, key: 303030303031 }
  key: 32 { type: Leaf, key: 303030303032 }
  key: 33 { type: Leaf, key: 303030303033 }
  key: 34 { type: Leaf, key: 303030303034 }
 }
 key: 31 { type: Leaf, key: 303031303035 }
}
```

* ART: Fix lower bound search within a node's children

Previously when doing a lower bound search on a node where the prefix was equal
and there was no child with an equal key chunk, we would move to the first
child. The correct thing to do is to move to the first _larger_ child.

* ART: Fix lower bound for equal prefix but smaller key

Previously we would assume a leaf had a key equal to the search key when the
parent prefix was equal, however this is not always the case.