Improve performance of custom-iterator __getitem__ (#1096)

Flipping the order of `Slice` and `Int` in `SliceOrInt` so that `Int`
comes first means that the `FromPyObject` derivation will then try `Int`
first, which is the correct variant in like 99.9% of uses of the struct.
This has the impact of improving int `__getitem__` times in the custom
iterators by about 3x (from 205ns to 61ns on my machine), which has
knock-on effects for the implicit iterators Python defines for these
classes.

src/iterators.rs

@@ -411,8 +411,8 @@ trait PyGCProtocol {
 
 #[derive(FromPyObject)]
 enum SliceOrInt<'a> {
-    Slice(&'a PySlice),
     Int(isize),
+    Slice(&'a PySlice),
 }
 
 trait PyConvertToPyArray {