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.h

@@ -0,0 +1,15 @@
+#ifndef LIGHTNING_TOOLS_TEST_ENUM_H
+#define LIGHTNING_TOOLS_TEST_ENUM_H
+#include <ccan/short_types/short_types.h>
+#include <stdlib.h>
+
+enum test_enum {
+	TEST_ONE,
+	TEST_TWO,
+};
+
+void towire_test_enum(u8 **pptr, const enum test_enum test_enum);
+enum test_enum fromwire_test_enum(const u8 **cursor, size_t *max);
+void printwire_test_enum(const char *fieldname, const enum test_enum *test_enum);
+
+#endif /* LIGHTNING_TOOLS_TEST_ENUM_H */