feat: add VecSignedMessage type alias

[SimenB]

This provides similar utility as VecBox in dryocsecretbox, allowing the following:

let message: Vec<u8> = some_message_from_somewhere;

let secret_box = VecBox::from_bytes(message).unwrap();

let message = secret_box.decrypt_to_vec(&nonce, &secret_key).unwrap();

let signed_message = VecSignedMessage::from_bytes(&message).unwrap();

signed_message.verify(public_key).unwrap();

let (_signature, message) = signed_message.into_parts();

// use unpacked `message`.

without VecSignedMessage, the from_bytes call on SignedMessage needs type annotations.

(Caveat - while I've used libsodium in javascript before, I'm very new to writing Rust. So this might not make sense 😀)

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[codecov]

Codecov Report

Merging #46 (846de8a) into main (949891e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #46   +/-   ##
=======================================
  Coverage   65.19%   65.19%           
=======================================
  Files          44       44           
  Lines        3166     3166           
=======================================
  Hits         2064     2064           
  Misses       1102     1102           

Impacted Files	Coverage Δ
src/sign.rs	15.78% <ø> (ø)

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