Rust review

[sneurlax]

• Remove unused functions as in 4e5ba7e
• Update authorship information
• Update changelog
• Update headers to reflect newest changes (like run_listener's definitions, etc)

Future work:

• Use official epic-wallet sources
• Doublecheck headers match FFI & usage
  Changes to API must be mirrored in at least both ios/Classes/FlutterLibepiccashPlugin.h and ios/Classes/SwiftFlutterLibepiccashPlugin.swift
• Catch panics and make sure Rust doesn’t unwind across the FFI boundary
  https://docs.rs/ffi_helpers/latest/ffi_helpers/panic/fn.catch_panic.html and attempt to catch errors as in https://docs.rs/ffi_helpers/latest/ffi_helpers/panic/fn.recover_panic_message.html
• Handle errors
  as in https://docs.rs/ffi_helpers/latest/ffi_helpers/error_handling/index.html
• Add more types around the FFI, change from eg. strings to an appropriate type
• Return full slate to Dart for safekeeping & rebroadcast
  returning the signature might be problematic because I'd think they'd expire--we'd need to make a new signature for the current epicbox server challenge. but maybe that isn't an issue if we just have the rust side do it all over, might be easy