Drop panic if rust-bitcoin adds a new Network

`rust-bitcoin` 0.30 added `#[non_exhaustive]` to the `Network`
enum, allowing them to "add support" for a new network type without
a major version change in the future. When upgrading, we added a
simple `unreachable` for the general match arm, which would break
in a minor version change of `rust-bitcoin`.

While it seems [possible rust-bitcoin will change
this](rust-bitcoin/rust-bitcoin#2225),
we still shouldn't ba panicking, which we drop here in favor of a
`debug_assert`ion, and a default value.

Author: TheBlueMatt

lightning-invoice/src/lib.rs

@@ -407,7 +407,10 @@ impl From<Network> for Currency {
 			Network::Testnet => Currency::BitcoinTestnet,
 			Network::Regtest => Currency::Regtest,
 			Network::Signet => Currency::Signet,
-			_ => unreachable!(),
+			_ => {
+				debug_assert!(false, "Need to handle new rust-bitcoin network type");
+				Currency::Regtest
+			},
 		}
 	}
 }