rustdoc: Prevent infinite recursion when inlining

Cyclic pub-use chains triggered infinite recursion, and this commit adds a hash
set to guard against cyclic recursion. This will cause one of the reexports to
render as a `pub use` instead of inlining the documentation.

Closes #16274
rustdoc: Don't try to inline the crate root

Fixes other test cases found in #16274
libnative/io: datasync() wrongly called fsync().
liblibc and libnative: send() should use const buffers.
Fix iterator doc

OrdIterator: the doc says that values must implement `PartialOrd`, while the implementation is only for `Ord` values. It looks like this initially got out of sync in 4e1c215. Removed the doc sentence entirely since it seems redundant.

MultiplicativeIterator: Fixed weird sentence.
Rename `begin_unwind` lang item to `fail_fmt`, refs #16114
Rename `core::failure::begin_unwind` to `fail_impl`, refs #16114
Rename `fail_` lang item to `fail`, closes #16114
Rename `begin_unwind_string` to `fail_str`, refs #16114
Beef up Default documentation
implement entry API for HashMap

Deprecates the `find_or_*` family of "internal mutation" methods on `HashMap` in
favour of the "external mutation" Entry API as part of RFC 60. Part of #17320,
although this still needs to be done on the rest of the maps, they don't have
any internal mutation methods defined, so they can be done without deprecating
or breaking anything. Work on `BTree`'s is part of the complete rewrite in #17334.

The implemented API deviates from the API described in the RFC in two key places:

* `VacantEntry.set` yields a mutable reference to the inserted element to avoid code
duplication where complex logic needs to be done *regardless* of whether the entry
was vacant or not.
* `OccupiedEntry.into_mut` was added so that it is possible to return a reference
into the map beyond the lifetime of the Entry itself, providing functional parity
to `VacantEntry.set`.

This allows the full find_or_insert functionality to be implemented using this API.
A PR will be submitted to the RFC to amend this.

[breaking-change]
handling fallout from entry api
auto merge of #17378 : Gankro/rust/hashmap-entry, r=aturon

Deprecates the `find_or_*` family of "internal mutation" methods on `HashMap` in
favour of the "external mutation" Entry API as part of RFC 60. Part of #17320,
but this still needs to be done on the rest of the maps. However they don't have
any internal mutation methods defined, so they can be done without deprecating
or breaking anything. Work on `BTree` is part of the complete rewrite in #17334.

The implemented API deviates from the API described in the RFC in two key places:

* `VacantEntry.set` yields a mutable reference to the inserted element to avoid code
duplication where complex logic needs to be done *regardless* of whether the entry
was vacant or not.
* `OccupiedEntry.into_mut` was added so that it is possible to return a reference
into the map beyond the lifetime of the Entry itself, providing functional parity
to `VacantEntry.set`.

This allows the full find_or_insert functionality to be implemented using this API.
A PR will be submitted to the RFC to amend this.

[breaking-change]
auto merge of #17428 : fhahn/rust/issue-16114-rename-begin-unwind-2, r=alexcrichton

This is a PR for #16114 and includes to following things:

* Rename `begin_unwind` lang item to `fail_fmt`
*  Rename `core::failure::begin_unwind` to `fail_impl`
* Rename `fail_` lang item to `fail`
auto merge of #17492 : alexcrichton/rust/issue-16274, r=aturon

Details in the commits.
auto merge of #17497 : nodakai/rust/libnative-misc-fixes, r=alexcrichton

libnative/io: datasync() wrongly called fsync().
liblibc and libnative: send() should use const buffers.
auto merge of #17498 : coyotebush/rust/iter-doc, r=huonw

OrdIterator: the doc says that values must implement `PartialOrd`, while the implementation is only for `Ord` values. It looks like this initially got out of sync in 4e1c215. Removed the doc sentence entirely since it seems redundant.

MultiplicativeIterator: Fixed weird sentence.
auto merge of #17455 : steveklabnik/rust/document_default, r=alexcrichton

Given that the `Default` module is now stable, it might as well get good docs.