Add is_expired_no_std to Offer

This was available for OfferContents but not an Offer so dependent
projects could not access it.

lightning/src/offers/offer.rs

@@ -468,6 +468,11 @@ impl Offer {
 		self.contents.is_expired()
 	}
 
+	/// Whether the offer has expired given the duration since the Unix epoch.
+	pub fn is_expired_no_std(&self, duration_since_epoch: Duration) -> bool {
+		self.contents.is_expired_no_std(duration_since_epoch)
+	}
+
 	/// Returns whether the given quantity is valid for the offer.
 	pub fn is_valid_quantity(&self, quantity: u64) -> bool {
 		self.contents.is_valid_quantity(quantity)