This matches std::collections

Closes #22181, #27779

FusedIterator is a marker trait that promises that the implementing
iterator continues to return `None` from `.next()` once it has returned
`None` once (and/or `.next_back()`, if implemented).

The effects of FusedIterator are already widely available through
`.fuse()`, but with stable `FusedIterator`, stable Rust users can
implement this trait for their iterators when appropriate.

The docs currently say, "If the closure returns false, it will try
again, and call the closure on the next element."  But this happens
regardless of whether the closure returns true or false.

Thew `_raw` prefix is included because the fact that `Box`’s ownership
semantics are "dissolved" or recreated seem more important than the exact
parameter type or return type.

`Shared` is now a deprecated `type` alias.

CC https://github.com/rust-lang/rust/issues/27730#issuecomment-352800629

This reverts commit 00acdbd5.

Relates to rust-lang/rfcs#2140 - drain_filter for all collections

`drain_filter` is implemented instead of `LinkedList::remove_if` based
on review feedback.

This commit removes the `rand` crate from the standard library facade as
well as the `__rand` module in the standard library. Neither of these
were used in any meaningful way in the standard library itself. The only
need for randomness in libstd is to initialize the thread-local keys of
a `HashMap`, and that unconditionally used `OsRng` defined in the
standard library anyway.

The cruft of the `rand` crate and the extra `rand` support in the
standard library makes libstd slightly more difficult to port to new
platforms, namely WebAssembly which doesn't have any randomness at all
(without interfacing with JS). The purpose of this commit is to clarify
and streamline randomness in libstd, focusing on how it's only required
in one location, hashmap seeds.

Note that the `rand` crate out of tree has almost always been a drop-in
replacement for the `rand` crate in-tree, so any usage (accidental or
purposeful) of the crate in-tree should switch to the `rand` crate on
crates.io. This then also has the further benefit of avoiding
duplication (mostly) between the two crates!

remove FIXME(#13101) since `assert_receiver_is_total_eq` stays.
remove FIXME(#19649) now that stability markers render.
remove FIXME(#13642) now the benchmarks were moved.
remove FIXME(#6220) now that floating points can be formatted.
remove FIXME(#18248) and write tests for `Rc<str>` and `Rc<[u8]>`
remove reference to irelevent issues in FIXME(#1697, #2178...)
update FIXME(#5516) to point to getopts issue 7
update FIXME(#7771) to point to RFC 628
update FIXME(#19839) to point to issue 26925

Like #43008 (f6689991), but _much more aggressive_.

* Added links where possible (limited because of facading)
* Changed references to methods from `foo()` to `foo` in module docs
* Changed references to methods from `HashMap::foo` to just `foo` in
  top-level docs for `HashMap` and the `default` doc for `DefaultHasher`
* Various small other fixes

This greatly improves consistency.

part of https://github.com/rust-lang/rust/issues/40912

[]\n() is not actually a link.

CC #34761