Add copy node function

[HeadHunter483]

This pull requests adds (*Node).CopyFromNode(*Root, *Node) function. The function copies the data from one node to another. If the src node has related nodes (e.g., next, nodes) they are copied as well to a matching fields of dst node. The first parameter of the function must be dst node's root so it utilizes its decoder node pool to avoid problems with GC.

Fixes #22