Function unseal 

pub fn unseal(
    session: &mut Session<Secure>,
    parsed: &ParsedPacket<'_>,
    raw: &[u8],
) -> Result<Vec<u8>, Error>

Verify the MAC on parsed and (if SCS_17/18) decrypt the DATA payload.

The caller is expected to first run ParsedPacket::parse and verify that a crate::packet::ScsType is present and is one of SCS_15..=18.