tlv: Add a typesafe serialization function for tlv namespaces

This is the counterpart to the typesafe deserialization function implemented
in an earlier commit.

tools/gen/header_template

@@ -66,7 +66,27 @@ struct ${tlv.struct_name()} {
 
 % for tlv in tlvs.values():
 struct ${tlv.struct_name()} *${tlv.struct_name()}_new(const tal_t *ctx);
-bool fromwire_${tlv.name}(const u8 **cursor, size_t *max, struct ${tlv.struct_name()} *record);
+
+/**
+ * Deserialize a TLV stream for the ${tlv.name} namespace.
+ *
+ * This function will parse any TLV stream, as long as the type, length and
+ * value fields are formatted correctly. Fields that are not known in the
+ * current namespace are stored in the `fields` member. Validity can be
+ * checked using ${tlv.name}_is_valid.
+ */
+bool fromwire_${tlv.name}(const u8 **cursor, size_t *max,
+			  struct ${tlv.struct_name()} * record);
+
+/**
+ * Serialize a TLV stream for the ${tlv.name} namespace.
+ *
+ * This function only considers known fields from the ${tlv.name} namespace,
+ * and will ignore any fields that may be stored in the `fields` member. This
+ * ensures that the resulting stream is valid according to
+ * `${tlv.name}_is_valid`.
+ */
+void towire_${tlv.name}(u8 **pptr, const void *record);
 
 /**
  * Check that the TLV stream is valid.

tools/gen/impl_template

@@ -218,6 +218,36 @@ ${static}const struct tlv_record_type tlvs_${tlv.name}[] = {
     % endfor
 };
 
+void towire_${tlv.name}(u8 **pptr,
+			const void *record)
+{
+	size_t num_types = ${len(tlv.messages)};
+	const struct tlv_record_type *types = tlvs_${tlv.name};
+	if (!record)
+		return;
+
+	for (size_t i = 0; i < num_types; i++) {
+		u8 *val;
+		if (i != 0)
+			assert(types[i].type > types[i-1].type);
+		val = types[i].towire(NULL, record);
+		if (!val)
+			continue;
+
+		/* BOLT #1:
+		 *
+		 * The sending node:
+		 ...
+		 *  - MUST minimally encode `type` and `length`.
+		 */
+		towire_bigsize(pptr, types[i].type);
+		towire_bigsize(pptr, tal_bytelen(val));
+		towire(pptr, val, tal_bytelen(val));
+		tal_free(val);
+	}
+}
+
+
 bool fromwire_${tlv.name}(const u8 **cursor, size_t *max, struct ${tlv.struct_name()} *record)
 {
 	size_t num_types = ${len(tlv.messages)};