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 caches the payment Invoice so that it can be used
when retrying the payment. It is parameterized by a Payer and Router for
ease of customization and testing.

Implement EventHandler for InvoicePayer as a decorator that intercepts
PaymentFailed events and retries that payment using the cached Invoice.
It delegates to the decorated EventHandler after retries have been
exhausted and for other events.

Author: jkczyz

lightning-invoice/Cargo.toml

@@ -13,6 +13,7 @@ bech32 = "0.8"
 lightning = { version = "0.0.100", path = "../lightning" }
 secp256k1 = { version = "0.20", features = ["recovery"] }
 num-traits = "0.2.8"
+bitcoin = "0.27"
 bitcoin_hashes = "0.10"
 
 [dev-dependencies]

lightning-invoice/src/lib.rs

@@ -15,11 +15,13 @@
 //!   * 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;
 extern crate bitcoin_hashes;
-extern crate lightning;
+#[macro_use] extern crate lightning;
 extern crate num_traits;
 extern crate secp256k1;