PaymentContext is already stored in ClaimablePayment via PaymentPurpose,
so there is no need to repeat it in each ClaimableHTLC via OnionPayload.
This avoids cloning the PaymentContext each time an HTLC is received,
other than for the first HTLC for a payment.

Event::PaymentClaimable and Event::PaymentClaimed already contain the
paid amount, so there's no need to included the requested amount in
InvoiceRequestFields.

InvoiceRequestFeatures may contain a large, odd bit. Including this in
InvoiceRequestFields, which is in each BlindedPath of a Bolt12Invoice,
could cause the invoice's onion message to exceed the maximum size. The
features are already checked before sending an invoice.