Rewrite Invoice's interface in terms of msats

InvoiceBuilder's interface was changed recently to work in terms of
msats. Update Invoice's interface to return the amount in msats, too,
and make amount_pico_btc private.

Author: jkczyz

lightning-invoice/src/lib.rs

@@ -1219,8 +1219,13 @@ impl Invoice {
 		self.signed_invoice.currency()
 	}
 
+	/// Returns the amount if specified in the invoice as millisatoshis.
+	pub fn amount_milli_satoshis(&self) -> Option<u64> {
+		self.signed_invoice.amount_pico_btc().map(|v| v / 10)
+	}
+
 	/// Returns the amount if specified in the invoice as pico <currency>.
-	pub fn amount_pico_btc(&self) -> Option<u64> {
+	fn amount_pico_btc(&self) -> Option<u64> {
 		self.signed_invoice.amount_pico_btc()
 	}
 }
@@ -1867,6 +1872,7 @@ mod test {
 		assert!(invoice.check_signature().is_ok());
 		assert_eq!(invoice.tagged_fields().count(), 10);
 
+		assert_eq!(invoice.amount_milli_satoshis(), Some(123));
 		assert_eq!(invoice.amount_pico_btc(), Some(1230));
 		assert_eq!(invoice.currency(), Currency::BitcoinTestnet);
 		assert_eq!(