Tree implementations that support extracting internal nodes and selector suffixes

[i-norden]

Related to #16. We need to extract internal nodes (HAMT and AMT nodes that do not contain a value, only a reference to a child node). The existing tree implementations (HAMT, AMT) have ForEach (iteration) methods that only trigger callbacks on leaf nodes. Additionally, we want our tree implementations to allow us to extract the path/position context while performing these traversals in order to extract the selector suffixes all leaf nodes.

We also need these tree implementations to support concurrent iteration of the trees.