Add InvoicePayer for retrying failed payments

When a payment fails, it's useful to retry the payment once the network
graph and channel scores are updated. InvoicePayer is a utility for
making payments which will retry any failed payment paths for a payment
up to a configured number of total attempts. It is parameterized by a
Payer and Router for ease of customization and testing.

Implement EventHandler for InvoicePayer as a decorator that intercepts
PaymentPathFailed events and retries that payment using the parameters
from the event. It delegates to the decorated EventHandler after retries
have been exhausted and for other events.

Author: jkczyz

lightning-invoice/src/lib.rs

@@ -15,11 +15,12 @@
 //!   * For parsing use `str::parse::<Invoice>(&self)` (see the docs of `impl FromStr for Invoice`)
 //!   * For constructing invoices use the `InvoiceBuilder`
 //!   * For serializing invoices use the `Display`/`ToString` traits
+pub mod payment;
 pub mod utils;
 
 extern crate bech32;
 extern crate bitcoin_hashes;
-extern crate lightning;
+#[macro_use] extern crate lightning;
 extern crate num_traits;
 extern crate secp256k1;