Add more info in internal node

AMT Internal nodes should add more contextual info to make them understandable independently.

• the base of their key (to understand which time period they concern and their position in the AMT)
• the count of elements under them (to quickly get a scale of the tree)
• the size in bits under them (more difficult because DAG can includes multiples times the same element but still useful)
• an optional signature to tell the data under them has been verified ?

However, they should remain as light as possible (and use dag-pb or something else than dagcbor).