Impl Base AMP in the receive pipeline and expose payment_secret

Impl Base AMP in the receive pipeline and expose payment_secret

Sadly a huge diff here, but almost all of it is changing the method
signatures for sending/receiving/failing HTLCs and the
PaymentReceived event, which all now need to expose an
Option<[u8; 32]> for the payment_secret.

It doesn't yet properly fail back pending HTLCs when the full AMP
payment is never received (which should result in accidental
channel force-closures). Further, as sending AMP payments is not
yet supported, the only test here is a simple single-path payment
with a payment_secret in it.