Add Borsh serialization support

[sunsided]

Adds support for the UUID crate's Borsh serialization feature:

#[test]
fn test_serialize() {
    let uuid_str = "f9168c5e-ceb2-4faa-b6bf-329bf39fa1e4";
    let sg = ShortGuid::from_str(uuid_str).unwrap();
    let sg_bytes = sg.as_bytes().to_vec();
    let borsh_bytes = borsh::to_vec(&sg).unwrap();
    assert_eq!(sg_bytes, borsh_bytes);
}

#[test]
fn test_deserialize() {
    let uuid_str = "f9168c5e-ceb2-4faa-b6bf-329bf39fa1e4";
    let sg = ShortGuid::from_str(uuid_str).unwrap();
    let sg_bytes = sg.as_bytes().to_vec();
    let deserialized = borsh::from_slice::<Uuid>(&sg_bytes).unwrap().to_string();
    assert_eq!(uuid_str, deserialized);
}

This also defaults the (default-active) randomizer to the fast-rng feature.

[pophilpo]

LGTM 👍