Queryable Sub-Trees

I see that the implementation is recursive, but this isn't exposed in the public API.

For example:
- `Get(k []byte)` is defined on `*Node`, but the only Node that is publicly accessible is the root node. It would be powerful to be able to make queries relative to any Node (e.g. `Get("xyz")` on the "abc" Node finding "abcxyz").
- The docs state "Iterator is used to return an iterator at the given node to walk the tree", but the only possible "given node" is the root node.

I might be able to figure it out reverse engineering the code, but I'm hoping the maintainers know whether this is something that is (a) already supported; just need to provide public access to non-root nodes, (b) not supported but could be with some reasonalble changes or \(c\) technically not possible given the current implementation.

This is a great library. I'm trying to do some great open source code with it.