Polish binary tree implementations

[rjl493456442]

• Fix node size in SerializeNode
• Track depth in node.Insert
• Copy the key-encoding logic from verkle, making it self-contained

[gballet]

Thanks for catching the encoding bug. For the record, I think that adding the depth to Insert is exposing an internal field externally, and that's not generally a good thing. But it doesn't change the correctness, so why not. Regarding the move of key functions from utils go` bintrie, this is ok for now, but it will break some unmerged code because it will create a cyclic dependency. But I'll move the file back when we get there, it makes sense to have it here for now.