Reuse types from mina-signer and proof-systems instead of reimplementing

[dannywillems]

Description

Several types are reimplemented when they could be reused from mina-signer or proof-systems.

Locations

• mina_compat.rs:95 - use mina_signer::Signature directly instead of wrapper
• network_id.rs:4 - use NetworkId from proof-systems
• hasher.rs:2,41-42 - duplicates code from mina-signer