Check expr type for ;step in s![] at compile time

Before, if the `expr;step` notation was used with an index as the
`expr`, e.g. `s![4;2]`, the macro would panic at runtime. Now, it
errors at compile time. This is accomplished by converting the `expr`
to a `Slice` before converting it into the final `AxisSliceInfo`.

Author: jturner314

src/slice.rs

@@ -880,7 +880,9 @@ macro_rules! s(
     };
     // convert range/index/new-axis and step into AxisSliceInfo
     (@convert $r:expr, $s:expr) => {
-        <$crate::AxisSliceInfo as ::std::convert::From<_>>::from($r).step_by($s as isize)
+        <$crate::AxisSliceInfo as ::std::convert::From<_>>::from(
+            <$crate::Slice as ::std::convert::From<_>>::from($r).step_by($s as isize)
+        )
     };
     ($($t:tt)*) => {
         $crate::s![@parse