Auto merge of #41437 - cuviper:remove-unstable-deprecated, r=alexcrichton

Remove items that are unstable and deprecated

This removes unstable items that have been deprecated for more than one cycle.

- Since 1.16.0, `#![feature(enumset)]`
    - All of `mod collections::enum_set`
- Since 1.15.0, `#![feature(borrow_state)]`
    - `cell::BorrowState`
    - `RefCell::borrow_state()`
- Since 1.15.0, `#![feature(is_unique)]`
    - `Rc::is_unique()` (made private like `Arc::is_unique()`)
- Since 1.15.0, `#![feature(rc_would_unwrap)]`
    - `Rc::would_wrap()`
- Since 1.13.0, `#![feature(binary_heap_extras)]`
    - `BinaryHeap::push_pop()`
    - `BinaryHeap::replace()`
- Since 1.12.0, `#![feature(as_unsafe_cell)]`
    - `Cell::as_unsafe_cell()`
    - `RefCell::as_unsafe_cell()`
- Since 1.12.0, `#![feature(map_entry_recover_keys)]`
    - `btree_map::OccupiedEntry::remove_pair()`
    - `hash_map::OccupiedEntry::remove_pair()`
- Since 1.11.0, `#![feature(float_extras)]`
    - `Float::nan()`
    - `Float::infinity()`
    - `Float::neg_infinity()`
    - `Float::neg_zero()`
    - `Float::zero()`
    - `Float::one()`
    - `Float::integer_decode()`
    - `f32::integer_decode()`
    - `f32::ldexp()`
    - `f32::frexp()`
    - `f32::next_after()`
    - `f64::integer_decode()`
    - `f64::ldexp()`
    - `f64::frexp()`
    - `f64::next_after()`
- Since 1.11.0, `#![feature(zero_one)]`
    - `num::Zero`
    - `num::One`

Author: bors

src/doc/unstable-book/src/SUMMARY.md

@@ -102,11 +102,8 @@
     - [alloc_system](library-features/alloc-system.md)
     - [alloc](library-features/alloc.md)
     - [as_c_str](library-features/as-c-str.md)
-    - [as_unsafe_cell](library-features/as-unsafe-cell.md)
     - [ascii_ctype](library-features/ascii-ctype.md)
-    - [binary_heap_extras](library-features/binary-heap-extras.md)
     - [binary_heap_peek_mut_pop](library-features/binary-heap-peek-mut-pop.md)
-    - [borrow_state](library-features/borrow-state.md)
     - [box_heap](library-features/box-heap.md)
     - [c_void_variant](library-features/c-void-variant.md)
     - [char_escape_debug](library-features/char-escape-debug.md)
@@ -131,14 +128,12 @@
     - [derive_clone_copy](library-features/derive-clone-copy.md)
     - [derive_eq](library-features/derive-eq.md)
     - [discriminant_value](library-features/discriminant-value.md)
-    - [enumset](library-features/enumset.md)
     - [error_type_id](library-features/error-type-id.md)
     - [exact_size_is_empty](library-features/exact-size-is-empty.md)
     - [fd](library-features/fd.md)
     - [fd_read](library-features/fd-read.md)
     - [fixed_size_array](library-features/fixed-size-array.md)
     - [float_bits_conv](library-features/float-bits-conv.md)
-    - [float_extras](library-features/float-extras.md)
     - [flt2dec](library-features/flt2dec.md)
     - [fmt_flags_align](library-features/fmt-flags-align.md)
     - [fmt_internals](library-features/fmt-internals.md)
@@ -158,15 +153,13 @@
     - [io_error_internals](library-features/io-error-internals.md)
     - [io](library-features/io.md)
     - [ip](library-features/ip.md)
-    - [is_unique](library-features/is-unique.md)
     - [iter_rfind](library-features/iter-rfind.md)
     - [libstd_io_internals](library-features/libstd-io-internals.md)
     - [libstd_sys_internals](library-features/libstd-sys-internals.md)
     - [libstd_thread_internals](library-features/libstd-thread-internals.md)
     - [linked_list_extras](library-features/linked-list-extras.md)
     - [lookup_host](library-features/lookup-host.md)
     - [manually_drop](library-features/manually-drop.md)
-    - [map_entry_recover_keys](library-features/map-entry-recover-keys.md)
     - [mpsc_select](library-features/mpsc-select.md)
     - [n16](library-features/n16.md)
     - [never_type_impls](library-features/never-type-impls.md)
@@ -189,7 +182,6 @@
     - [rand](library-features/rand.md)
     - [range_contains](library-features/range-contains.md)
     - [raw](library-features/raw.md)
-    - [rc_would_unwrap](library-features/rc-would-unwrap.md)
     - [retain_hash_collection](library-features/retain-hash-collection.md)
     - [reverse_cmp_key](library-features/reverse-cmp-key.md)
     - [rt](library-features/rt.md)
@@ -225,5 +217,3 @@
     - [windows_handle](library-features/windows-handle.md)
     - [windows_net](library-features/windows-net.md)
     - [windows_stdio](library-features/windows-stdio.md)
-    - [zero_one](library-features/zero-one.md)
->>>>>> Add top level sections to the Unstable Book.