make RangeArgument #[fundamental]

This is necessary so that crates outside of libcore (e.g.
libcollections) are allowed to write both `impl Index<usize> for _` and
`impl<R: RangeArgument<usize>> Index<R> for _` without getting overlap
errors (essentially, to assume that `usize: !RangeArgument<usize>`).

Author: durka

src/libcore/ops.rs

@@ -1896,6 +1896,7 @@ pub trait BoxPlace<Data: ?Sized> : Place<Data> {
 #[unstable(feature = "collections_range",
        reason = "waiting for dust to settle on inclusive ranges",
        issue = "30877")]
+#[fundamental]
 pub trait RangeArgument<T> {
     /// Start index (inclusive)
     ///