simplify load_example_wallet match

[zancas]

This successive match makes the code slightly less repetitive and is a bit easier to read I think since it separates the match of the network from the match of the seed, and handles the seed match inside the network match as a separate and subsequent concern.

[zancas]

example_wallet_base and example_wallet_address could be similarly refactored into a cleaner hierarchical separation of concerns. In fact, this pattern would extend the existing pattern for the pool match in example_wallet_address.