This commit brings the `Error` trait in line with the [Error interoperation
RFC](https://github.com/rust-lang/rfcs/pull/201) by adding downcasting,
which has long been intended. This change means that for any `Error`
trait objects that are `'static`, you can downcast to concrete error
types.

To make this work, it is necessary for `Error` to inherit from
`Reflect` (which is currently used to mark concrete types as "permitted
for reflection, aka downcasting"). This is a breaking change: it means
that impls like

```rust
impl<T> Error for MyErrorType<T> { ... }
```

must change to something like

```rust
impl<T: Reflect> Error for MyErrorType<T> { ... }
```

except that `Reflect` is currently unstable (and should remain so for
the time being). For now, code can instead bound by `Any`:

```rust
impl<T: Any> Error for MyErrorType<T> { ... }
```

which *is* stable and has `Reflect` as a super trait. The downside is
that this imposes a `'static` constraint, but that only
constrains *when* `Error` is implemented -- it does not actually
constrain the types that can implement `Error`.

[breaking-change]

Make whitespace consistent

This adds some missing punctuation and converts uses of "Gets" to
"Returns". This sounds better to my ear, but more importantly is
more consistent with the documentation from other files.

This adds some missing punctuation, adds a missing word, and
corrects a bug in the description of `send_to`, which actually
returns the number of bytes written on success.

Fixes #24925.

Currently if a standard I/O handle is set to inherited on Windows, no action is
taken and the slot in the process information description is set to
`INVALID_HANDLE_VALUE`. Due to our passing of `STARTF_USESTDHANDLES`, however,
this means that the handle is actually set to nothing and if a child tries to
print it will generate an error.

This commit fixes this behavior by explicitly creating stdio handles to be
placed in these slots by duplicating the current process's I/O handles. This is
presumably what previously happened silently by using a file-descriptor-based
implementation instead of a `HANDLE`-centric implementation.

Along the way this cleans up a lot of code in `Process::spawn` for Windows by
ensuring destructors are always run, using more RAII, and limiting the scope of
`unsafe` wherever possible.

This is the last remaining portion of #24796

This is served by stability markers.

This commit is an implementation of [RFC 1044][rfc] which adds additional
surface area to the `std::fs` module. All new APIs are `#[unstable]` behind
assorted feature names for each one.

[rfc]: https://github.com/rust-lang/rfcs/pull/1044

The new APIs added are:

* `fs::canonicalize` - bindings to `realpath` on unix and
  `GetFinalPathNameByHandle` on windows.
* `fs::symlink_metadata` - similar to `lstat` on unix
* `fs::FileType` and accessor methods as `is_{file,dir,symlink}`
* `fs::Metadata::file_type` - accessor for the raw file type
* `fs::DirEntry::metadata` - acquisition of metadata which is free on Windows
  but requires a syscall on unix.
* `fs::DirEntry::file_type` - access the file type which may not require a
  syscall on most platforms.
* `fs::DirEntry::file_name` - access just the file name without leading
  components.
* `fs::PathExt::symlink_metadata` - convenience method for the top-level
  function.
* `fs::PathExt::canonicalize` - convenience method for the top-level
  function.
* `fs::PathExt::read_link` - convenience method for the top-level
  function.
* `fs::PathExt::read_dir` - convenience method for the top-level
  function.
* `std::os::raw` - type definitions for raw OS/C types available on all
  platforms.
* `std::os::$platform` - new modules have been added for all currently supported
  platforms (e.g. those more specific than just `unix`).
* `std::os::$platform::raw` - platform-specific type definitions. These modules
  are populated with the bare essentials necessary for lowing I/O types into
  their raw representations, and currently largely consist of the `stat`
  definition for unix platforms.

This commit also deprecates `Metadata::{modified, accessed}` in favor of
inspecting the raw representations via the lowering methods of `Metadata`.

Inspecting the current thread's info may not always work due to the TLS value
having been destroyed (or is actively being destroyed). The code for printing
a panic message assumed, however, that it could acquire the thread's name
through this method.

Instead this commit propagates the `Option` outwards to allow the
`std::panicking` module to handle the case where the current thread isn't
present.

While it solves the immediate issue of #24313, there is still another underlying
issue of panicking destructors in thread locals will abort the process.

Closes #24313

Don't need so much manual #[doc(hidden)] and #[unstable] as much of it is
inherited!

Statically linked executables do not succeed (aka MUSL-based executables).

This commit modifies the standard library and its dependencies to link correctly
when built against MUSL. This primarily ensures that the right libraries are
linked against and when they're linked against they're linked against
statically.

Fix `make_command_line` for the case of backslashes at the end of an
argument requiring quotes.  We must encode the command and arguments
such that `CommandLineToArgvW` recovers them in the spawned process.
Simplify the logic by using a running count of backslashes as they
are encountered instead of looking ahead for quotes following them.

Extend `test_make_command_line` to additionally cover:

* a leading quote in an argument that requires quotes,
* a backslash before a quote in an argument that requires quotes,
* a backslash at the end of an argument that requires quotes, and
* a backslash at the end of an argument that does not require quotes.

Changes the style guidelines regarding unit tests to recommend using a
sub-module named "tests" instead of "test" for unit tests as "test"
might clash with imports of libtest.

These functions were intended to be introduced as `#[stable]` as a stable API
was deprecated in favor of them, but they just erroneously forgot the stability
attributes.

`ToCStr` was removed with `old_io` and the current method `as_os_str`
is inherent to `Path`, meaning there is no suitable trait bound that
could be used here.

Much of this code hasn't been updated in quite some time and this commit does a
small audit of the functionality:

* Implementation functions now centralize all functionality on a locally defined
  `Thread` type.
* The `detach` method has been removed in favor of a `Drop` implementation. This
  notably fixes leaking thread handles on Windows.
* The `Thread` structure is now appropriately annotated with `Send` and `Sync`
  automatically on Windows and in a custom fashion on Unix.
* The unsafety of creating a thread has been pushed out to the right boundaries
  now.

Closes #24442

In 8f5b5f94, `default::Default` was
added to the prelude, so these imports are no longer necessary.

This API was exercised in a few tests and mirrors the `from_str_radix`
functionality of the integer types.

This commit removes all the old casting/generic traits from `std::num` that are
no longer in use by the standard library. This additionally removes the old
`strconv` module which has not seen much use in quite a long time. All generic
functionality has been supplanted with traits in the `num` crate and the
`strconv` module is supplanted with the [rust-strconv crate][rust-strconv].

[rust-strconv]: https://github.com/lifthrasiir/rust-strconv

This is a breaking change due to the removal of these deprecated crates, and the
alternative crates are listed above.

[breaking-change]

Cleaning out more deprecated items

On Windows, when you create a symbolic link you must specify whether it
points to a directory or a file, even if it is created dangling, while
on Unix, the same symbolic link could point to a directory, a file, or
nothing at all.  Furthermore, on Windows special privilege is necessary
to use a symbolic link, while on Unix, you can generally create a
symbolic link in any directory you have write privileges to.

This means that it is unlikely to be able to use symbolic links purely
portably; anyone who uses them will need to think about the cross
platform implications.  This means that using platform-specific APIs
will make it easier to see where code will need to differ between the
platforms, rather than trying to provide some kind of compatibility
wrapper.

Furthermore, `soft_link` has no precedence in any other API, so to avoid
confusion, move back to the more standard `symlink` terminology.

Create a `std::os::unix::symlink` for the Unix version that is
destination type agnostic, as well as `std::os::windows::{symlink_file,
symlink_dir}` for Windows.

Because this is a stable API, leave a compatibility wrapper in
`std::fs::soft_link`, which calls `symlink` on Unix and `symlink_file`
on Windows, preserving the existing behavior of `soft_link`.