bolt-gen: handle enums

Add parsing know-how for enum fields. This is necessary for
internally defined wire generators. Enums are denoted by prefixing
the field with an `e:`.

Ex:

   msgdata,msg_name,field_name,e:enum_type,

tools/test/enum.c

@@ -0,0 +1,18 @@
+#include "enum.h"
+#include <stdio.h>
+
+void towire_test_enum(u8 **pptr, const enum test_enum test_enum)
+{
+	printf("this would have been the towire for enum %u\n", test_enum);
+}
+
+enum test_enum fromwire_test_enum(const u8 **cursor, size_t *max)
+{
+	printf("fromwire_test_enum at %ld\n", *max);
+	return TEST_ONE;
+}
+
+void printwire_test_enum(const char *fieldname, const enum test_enum *test_enum)
+{
+	printf("%u\n", *test_enum);
+}