Rename the `primitives.h` and `primitives.c` files to `wren_core.h` and `wren_core.c` respectively, reflecting the module's purpose of defining built-in classes and native methods. Update all includes, macro names (PRIMITIVE -> NATIVE, DEF_PRIMITIVE -> DEF_NATIVE, etc.), and function calls (wrenLoadCore -> wrenInitializeCore) across the codebase and Xcode project file to match the new naming convention.
