publishing and sharing audit files

[cdstanford]

Locally, all audit files are stored/overwritten to the same file location.
We may need a way to sign, publish, and version-number audits and associated metadata to share with other users and organizations.

This is related to #60.

This also relates to the problem we discussed today about tracking which portions of the code the user has audited or not -- since this information could be tracked in the audit files. An alternative would be to change our safety annotation model -- see audit_file.rs:

#[derive(Debug, Serialize, Deserialize, Clone, Copy, PartialEq, Eq)]
pub enum SafetyAnnotation {
    Skipped,
    Safe,
    Unsafe,
    CallerChecked,
}