Function compute_identity_root 

pub fn compute_identity_root(
    trace_id: &str,
    identity_route: &str,
    span_hashes: &[Sha256],
) -> Result<Sha256, HashError>

Computes the identity root.

identity_root = sha256(JCS({trace_id, identity_route, sorted(C_span_i)}))

This binds the trace to a specific identity while keeping the trace_set_root independent of identity.