Auto merge of #34530 - alexcrichton:stabilize-1.11, r=aturon

std: Stabilize APIs for the 1.11.0 release

Although the set of APIs being stabilized this release is relatively small, the
trains keep going! Listed below are the APIs in the standard library which have
either transitioned from unstable to stable or those from unstable to
deprecated.

Stable

* `BTreeMap::{append, split_off}`
* `BTreeSet::{append, split_off}`
* `Cell::get_mut`
* `RefCell::get_mut`
* `BinaryHeap::append`
* `{f32, f64}::{to_degrees, to_radians}` - libcore stabilizations mirroring past
  libstd stabilizations
* `Iterator::sum`
* `Iterator::product`

Deprecated

* `{f32, f64}::next_after`
* `{f32, f64}::integer_decode`
* `{f32, f64}::ldexp`
* `{f32, f64}::frexp`
* `num::One`
* `num::Zero`

Added APIs (all unstable)

* `iter::Sum`
* `iter::Product`
* `iter::Step` - a few methods were added to accomodate deprecation of One/Zero

Removed APIs

* `From<Range<T>> for RangeInclusive<T>` - everything about `RangeInclusive` is
  unstable

Closes #27739
Closes #27752
Closes #32526
Closes #33444
Closes #34152
cc #34529 (new tracking issue)

diff --cc src/libcoretest/lib.rs
Simple merge