FEAT: New syntax for node children and paths

[tturocy]

Constructing paths in an extensive game is currently quite tedious and verbose. To this end the following features are proposed:

1. Indexing into the children of a node will be done by action label rather than the label of the child node. This is a breaking change but should be seen as definitely an improvement (see e.g. the discussion in Ensure append_move and other functions to apply labels to nodes by default #553 for an example where the current behaviour was unanticipated)

2. Implement the operator + with LHS of a node and RHS of an action label producing a node.