Initial implementation of anonymous_pipe

Co-authored-by: Alphyr <47725341+a1phyr@users.noreply.github.com>
Co-authored-by: Jubilee <46493976+workingjubilee@users.noreply.github.com>
Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

library/std/src/sys/mod.rs

@@ -7,6 +7,8 @@ mod pal;
 
 mod personality;
 
+#[unstable(feature = "anonymous_pipe", issue = "127154")]
+pub mod anonymous_pipe;
 pub mod backtrace;
 pub mod cmath;
 pub mod exit_guard;