STRUCT_MEMBER macro
Return field in a `ProtobufCMessage` based on offset. Take a pointer to a `ProtobufCMessage` and find the field at the offset. Cast it to the passed type.
Syntax
#define STRUCT_MEMBER(member_type, struct_p, struct_offset) \
(*(member_type *) STRUCT_MEMBER_P((struct_p), (struct_offset)))