Enable all warnings during C compilation
CRuby: Expose current_bindings as Bindex.current_bindings

It is useful, if you want to directly access them.
CRuby: Clean unused argument in bx_exc_bindings
JRuby: Expose Bindex.current_bindings
Add a simple smoke test for Bindex.current_bindings
JRuby: Implement Exception#bindings in plain Java
Rubinius: Expose Bindex.current_bindings