Create new InvoiceFeatures object for Invoice-specific features

In the past we skipped doing this since invoice parsing occurs in a
different crate. However, we need to accept InvoiceFeatures in routing
now that we support MPP route collection, to detect if we can select
multiple paths or not. Further, we should probably take
rust-lightning-invoice as either a module or a subcrate in this repo.

Author: TheBlueMatt

lightning/src/ln/features.rs

@@ -140,6 +140,17 @@ mod sealed {
 		required_features: [],
 		optional_features: [],
 	});
+	define_context!(InvoiceContext {
+		required_features: [,,,],
+		optional_features: [
+			// Byte 0
+			,
+			// Byte 1
+			VariableLengthOnion | PaymentSecret,
+			// Byte 2
+			BasicMPP,
+		],
+	});
 
 	/// Defines a feature with the given bits for the specified [`Context`]s. The generated trait is
 	/// useful for manipulating feature flags.
@@ -256,9 +267,9 @@ mod sealed {
 		"Feature flags for `var_onion_optin`.");
 	define_feature!(13, StaticRemoteKey, [InitContext, NodeContext],
 		"Feature flags for `option_static_remotekey`.");
-	define_feature!(15, PaymentSecret, [InitContext, NodeContext],
+	define_feature!(15, PaymentSecret, [InitContext, NodeContext, InvoiceContext],
 		"Feature flags for `payment_secret`.");
-	define_feature!(17, BasicMPP, [InitContext, NodeContext],
+	define_feature!(17, BasicMPP, [InitContext, NodeContext, InvoiceContext],
 		"Feature flags for `basic_mpp`.");
 	define_feature!(27, ShutdownAnySegwit, [InitContext, NodeContext],
 		"Feature flags for `opt_shutdown_anysegwit`.");
@@ -323,6 +334,8 @@ pub type InitFeatures = Features<sealed::InitContext>;
 pub type NodeFeatures = Features<sealed::NodeContext>;
 /// Features used within a `channel_announcement` message.
 pub type ChannelFeatures = Features<sealed::ChannelContext>;
+/// Features used within an invoice.
+pub type InvoiceFeatures = Features<sealed::InvoiceContext>;
 
 impl InitFeatures {
 	/// Writes all features present up to, and including, 13.
@@ -359,6 +372,14 @@ impl InitFeatures {
 	}
 }
 
+impl InvoiceFeatures {
+	/// Converts `InvoiceFeatures` to `Features<C>`. Only known `InvoiceFeatures` relevant to
+	/// context `C` are included in the result.
+	pub(crate) fn to_context<C: sealed::Context>(&self) -> Features<C> {
+		self.to_context_internal()
+	}
+}
+
 impl<T: sealed::Context> Features<T> {
 	/// Create a blank Features with no features set
 	pub fn empty() -> Self {