Virtually the whole ruby core moved from RData to RTypeData, let's do so
ourselves too.
Basically the information typically passed through Data_Wrap_Struct is
now stored in a struct rb_data_type_t (mark and free functions). This
has the advantage that more information can be easily added, like the
name of the type, a custom data ponter, and more.
Data_Wrap_Struct is replaced with TypedData_Wrap_Struct, and the
information is stored in a struct rb_data_type_t, rather than passed
as arguments.
Check_Type is replaced with Check_TypedStruct, which is a wrapper for
rb_check_typeddata (with casts).