Update macros.rs

[nlsnightmare]

Updated macros af_print, join_many and eval to work with expressions instead of identifiers.
Before the update, code like this:

let a = arrayfire::randu(Dim4::new(&[3, 3, 1, 1]));
af_print!("A transposed: ", arrayfire::transpose(&a, false));

Would not compile, and should have been written like this:

let a = arrayfire::randu(Dim4::new(&[3, 3, 1, 1]));
let a_T =  arrayfire::transpose(&a, false);
af_print!("A transposed: ", a_T);

The update fixes this issue

[9prady9]

@nlsnightmare thank you!