API change: rename sons/father to next/predecessor in nodes

I think next/predecessor will be more intuitive than sons/father for the CFG (https://github.com/crytic/slither/blob/master/slither/core/cfg/node.py)

This will be a breaking change, but most likely not too many external code relies on this.

I would in favor to keep in parallel keep `sons` and `fathers` for a few iteration but making it clear that they are deprecated