Stabilize Index traits and most range notation

This commit marks as `#[stable]`:

* The `Index` and `IndexMut` traits. These are stabilized as taking the
  index itself *by reference*; after extensive discussion it was
  determined that this is a better match with our choices
  elsewhere (e.g. making comparison operators auto-reference), and that
  the use cases for by-value indices are better handled through
  `IndexSet`.

* The `Range`, `RangeFrom` and `RangeTo` structs, introduced for range
  notation.

* Various impls of `Index` and `IndexMut`.

The `FullRange` struct is left unstable as we may wish to rename it to
`RangeFull` in the future.

This commit also *removes* the `Step` trait in favor of direct
implementation of iterator traits on ranges for integers. The `Step`
trait was not a terribly useful factoring internally, and it is likely
that external integer types are best off implementing range iterators
directly. It was removed to simplify the API surface. We can always
reintroduce `Step` later if it turns out to be useful.

Due to this removal, this is a:

[breaking-change]

0 files changed, 0 insertions, 0 deletions