Add overloaded slice operations to the prelude

methods on slice types. However, the relevant traits were not included
in the prelude, meaning that you would have to manually import
`std::ops::Slice` to get these operations -- an unintended regression.

This commit adds the traits to the prelude, as is done with most other
operator traits.

Unfortunately, the trait `std::slice::Slice` (which defines an
`as_slice` method) was already included in the prelude. This trait is
renamed to `AsSlice`, which is a better name in any case.

In addition, because of both `AsSlice` and `Str` traits being included
in the prelude, both of which define `as_slice`, and both of which are
used for generic programming, this commit renames `ops::Slice::as_slice`
to `ops::Slice::as_slice_`. This is a stopgap solution; we'll need to
figure out a long-term story here later on.

Closes rust-lang#17710

Due to the renamings, this is a:

[breaking-change]