Use RBASIC_CLASS instead of RBASIC()->klass

I want to try out @csfrancis's [32-byte object patch](https://github.com/csfrancis/ruby/compare/csfrancis:17aa097cd1f927a4ecd0f00177f743d03844ae7a...compressed_objects?expand=1), but that removes the `klass` field from the `RBasic` struct so this extension fails to compile.

cc @tmm1