core: Use primitive indexing in slice's Index/IndexMut

[T]'s Index implementation is normally not used for indexing, instead
the compiler supplied indexing is used.

Use the compiler supplied version in Index/IndexMut.

This removes an inconsistency:

Compiler supplied bound check failures look like this:

thread 'main' panicked at 'index out of bounds: the len is 3 but the index is 4'

If you convince Rust to use the Index impl for slices, bounds check
failure looks like this instead:

thread 'main' panicked at 'assertion failed: index < self.len()'

The latter is used if you for example use Index generically::

   use std::ops::Index;
   fn foo<T: ?Sized>(x: &T) where T: Index<usize> { &x[4]; }

   foo(&[1, 2, 3][..])

src/test/run-fail/bounds-check-no-overflow.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern:assertion failed: index < self.len()
+// error-pattern:index out of bounds
 
 use std::usize;
 use std::mem::size_of;