Fix: SMT Node left_child and right_child return placeholder when key is zero sum

[bvrooman]

When retrieving a left child or right child from an SMT node, we must accommodate the case when the child is a placeholder. The placeholder node is a node with the zero sum key (000...). In the current design, all children must be present in the underlying data store; this includes placeholders. This assumes that the store will be initialized with a zero-sum key mapped to a placeholder, i.e.

...
let zero_sum_key = zero_sum()
assert!(storage[zero_sum_key].is_placeholder())

This has a few important drawbacks:

• The store must be initialized properly to handle this edge case. This means a storage write when initializing an SMT
• Reading a zero sum key incurs a round trip to the store, when the child can be resolved client side
• Deleting path nodes or side nodes may involve deleting a placeholder; this will leave the storage in an invalid state as no further placeholders can be loaded

These drawbacks are all neatly resolved by ensuring that a placeholder is retuned when encountering a zero sum key at the library level.