Function find_signed_user 

pub(crate) fn find_signed_user<'c>(
    cert: &'c OpenpgpCert,
    user_id: &UserId,
) -> Option<&'c SignedUser>

Finds the [SignedUser] that matches a user_id.

This object contains the valid (cryptographically and based on rpgpie policy) self-signatures over that user id.