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

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][..])