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

diff --cc src/libstd/io/net/tcp.rs
Simple merge

diff --cc src/libstd/os.rs
Simple merge

diff --cc src/libstd/sys/common/mod.rs
Simple merge

diff --cc src/libstd/sys/unix/mod.rs
index 7a5317b578d9346244e038de35b43617545cc13f,d800c766426033f3a5a5e5c75b7e3e8faaec443f..af2389051190d98b2566d2afa640f6931e3fb5d1
--- 1/src/libstd/sys/unix/mod.rs
--- 2/src/libstd/sys/unix/mod.rs
+++ b/src/libstd/sys/unix/mod.rs
@@@ -33,16 -33,16 +33,17 @@@ macro_rules! helper_init( (static $name
  ) )
  
  pub mod c;
+ pub mod ext;
  pub mod fs;
 +pub mod helper_signal;
  pub mod os;
 -pub mod tcp;
 -pub mod udp;
  pub mod pipe;
 -pub mod helper_signal;
  pub mod process;
 +pub mod tcp;
  pub mod timer;
 +pub mod thread_local;
  pub mod tty;
 +pub mod udp;
  
  pub mod addrinfo {
      pub use sys_common::net::get_host_addresses;

diff --cc src/libstd/sys/windows/mod.rs
index a785ccfe80465ff8891518d3ccdf70098ef9ba3f,33e7094612e3932f18ffdc385f0c25f4c58b1dd5..6b9555c52cec7cdb73dfc20954a23d1209f7901d
--- 1/src/libstd/sys/windows/mod.rs
--- 2/src/libstd/sys/windows/mod.rs
+++ b/src/libstd/sys/windows/mod.rs
@@@ -34,16 -34,16 +34,17 @@@ macro_rules! helper_init( (static $name
  ) )
  
  pub mod c;
+ pub mod ext;
  pub mod fs;
 +pub mod helper_signal;
  pub mod os;
 -pub mod tcp;
 -pub mod udp;
  pub mod pipe;
 -pub mod helper_signal;
  pub mod process;
 +pub mod tcp;
 +pub mod thread_local;
  pub mod timer;
  pub mod tty;
 +pub mod udp;
  
  pub mod addrinfo {
      pub use sys_common::net::get_host_addresses;