Update hash aliaces

[Fumuran]

To follow the other convention:

/// SMT node representation (32-byte Keccak256 hash)
pub type SmtNode = Keccak256Output;

/// Exit root representation (32-byte Keccak256 hash)
pub type ExitRoot = Keccak256Output;

We should:

• Create the aliases for the leaf value and CGI chain hash.
• Add a Keccak256Output::from_hex constructor.
• Move the Keccak256Output definition from claim_note.rs into utils.rs. ExitRoot and SmtNode aliases can stay in claim_note.rs.

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GlobalIndex struct duplicates a lot of the functionality of Keccak256Output, so we should rework the GlobalIndex struct into a trait which will extend Keccak256Output with the extra methods. These extended methods seem to be anyway used only for testing, so this new trait could have a cfg(test) attribute.

Originally posted by @mmagician in #2516 (comment)