fix: bind to next node if possible

[twoeths]

Motivation

• in setNodes apis, if both left and right are set then we can create a new parent instead of rebind 2 times (and waste one loop)
• this was implemented in ssz-js already https://github.com/ChainSafe/ssz/blob/7f5580c2ea69f9307300ddb6010a8bc7ce2fc471/packages/persistent-merkle-tree/src/tree.ts#L429

Description

• implement leftN and rightN in Path
• use while loop instead of for loop so that we can skip at any times
• if a node is left, and next node is its right now, create new parent and skip the next loop

part of #68

[wemeetagain]

Can you add a benchmark or run the existing benchmark (zbuild run bench_node) to see the effect that this PR makes?