Function identity_bound_at 

pub(crate) fn identity_bound_at(
    cert: &OpenpgpCert,
    sigs: &[Signature],
    reference_time: SystemTime,
) -> bool

Ensures that sigs contains a certifying signature that is valid at reference_time, for self-binding an identity to cert.

This assumes sigs contains validated self-signatures over a user id (which is guaranteed if they come from a Checked).

The creation time of cert is used as a lower bound for identity validity.