When using rb_data_type_struct to wrap a C struct, that C struct can contain VALUE references to other Ruby objects. If this is the case then one must also define dmark and optionally dcompact callbacks in order to allow these objects to be correctly handled by the GC. This is suboptimal as it requires GC related logic to be implemented by extension developers. This can be a cause of subtle bugs when references are not marked of updated correctly inside these callbacks. This commit provides an alternative approach, useful in the simple case where the C struct contains VALUE members (ie. there isn't any conditional logic, or data structure manipulation required to traverse these references). In this case references can be defined using a declarative syntax as a list of edges (or, pointers to references). A flag can be set on the rb_data_type_struct to notify the GC that declarative references are being used, and a list of those references can be assigned to the dmark pointer instead of a function callback, on the rb_data_type_struct. Macros are also provided for simple declaration of the reference list, and building edges. To avoid having to also find space in the struct to define a length for the references list, I've chosed to always terminate the references list with RUBY_REF_END - defined as UINTPTR_MAX. My assumption is that no single struct will ever be large enough that UINTPTR_MAX is actually a valid reference.