Provide the HTLCs that settled a payment.

Provide the HTLCs that settled a payment.

Creates a new `events::ClaimedHTLC` struct that contains the relevant
information about a claimed HTLC; e.g., the channel it arrived on, its ID, the
amount of the HTLC, the overall amount of the payment, etc. Adds appropriate
serialization support.

Adds a `Vec<events::ClaimedHTLC>` to the `ClaimingPayment`
structure. Populates this when creating the struct by converting the
`payment.htlcs` (which are `ClaimingHTLC` structs) into `event::ClaimedHTLC`
structs. This is a straightforward transformation.

Adds a `Vec<events::ClaimedHTLC>` to the `events::Event::PaymentClaimed`
enum. This is populated directly from the `ClaimingPayment`'s `htlcs` vec.

Fixes #2477.