sys: reveal std::io representation to sys module

This commit adds a `AsInner` trait to `sys_common` and provides
implementations on many `std::io` types. This is a building block for
exposing platform-specific APIs that hook into `std::io` types.
libs: add std::os::unix module

The new `std::os::unix` module exposes several extension traits
for extracting file descriptors from `std::io` types.
libs: add std::os::windows module

The new `std::os::windows` module exposes several extension traits
for extracting file descriptors, sockets, and handles from `std::io`
types.
Search for implemented kinds recursively on Trait types. Fixes #15155 and #13155.
Fixed "::::" appearing in module_path!()
Added test for module_path! fix
Deprecate MaybeOwned[Vector] in favor of Cow
auto merge of #19011 : ricky26/rust/trait_supertraits, r=nikomatsakis

It looks like currently kinds required by traits are not propagated when they are wrapped in a TyTrait. Additionally, in SelectionContext::builtin_bound, no attempt is made to check whether the target trait or its supertraits require the kind specified.

This PR alters SelectionContext::builtin_bound to examine all supertraits in the target trait's bounds recursively for required kinds.

Alternatively, the kinds could be added to the TyTrait upon creation (by just setting its builtin_bounds to the union of the bounds requested in this instance and the bounds required by the trait), this option may have less overhead during compilation but information is lost about which kinds were explicitly requested for this instance (vs those specified by traits/supertraits) would be lost.
remove deprecated stuff from std::fmt docs

Fixes #19209
auto merge of #19212 : steveklabnik/rust/doc_format_specifiers, r=alexcrichton

Fixes #19209
auto merge of #19169 : aturon/rust/fds, r=alexcrichton

This PR adds some internal infrastructure to allow the private `std::sys` module to access internal representation details of `std::io`.

It then exposes those details in two new, platform-specific API surfaces: `std::os::unix` and `std::os::windows`.

To start with, these will provide the ability to extract file descriptors, HANDLEs, SOCKETs, and so on from `std::io` types.

More functionality, and more specific platforms (e.g. `std::os::linux`) will be added over time.

Closes #18897
auto merge of #19252 : japaric/rust/cow, r=aturon

- Add `IntoCow` trait, and put it in the prelude
- Add `is_owned`/`is_borrowed` methods to `Cow`
- Add `CowString`/`CowVec` type aliases (to `Cow<'_, String, str>`/`Cow<'_, Vec, [T]>` respectively)
- `Cow` implements: `Show`, `Hash`, `[Partial]{Eq,Ord}`
- `impl BorrowFrom<Cow<'a, T, B>> for B`

[breaking-change]s:

- `IntoMaybeOwned` has been removed from the prelude
- libcollections: `SendStr` is now an alias to `CowString<'static>` (it was aliased to `MaybeOwned<'static>`)
- libgraphviz:
  - `LabelText` variants now wrap `CowString` instead of `MaybeOwned`
  - `Nodes` and `Edges` are now type aliases to `CowVec` (they were aliased to `MaybeOwnedVec`)
- libstd/path: `Display::as_maybe_owned` has been renamed to `Display::as_cow` and now returns a `CowString`
- These functions now accept/return `Cow` instead of `MaybeOwned[Vector]`:
  - libregex: `Replacer::reg_replace`
  - libcollections: `str::from_utf8_lossy`
  - libgraphviz: `Id::new`, `Id::name`, `LabelText::pre_escaped_content`
  - libstd: `TaskBuilder::named`

r? @aturon
auto merge of #19262 : murarth/rust/module-path-fix, r=jakub-

Closes #18859
/*! -> //!

Sister pull request of https://github.com/rust-lang/rust/pull/19288, but
for the other style of block doc comment.