Add yanix crate and replace nix with yanix in wasi-common (#649)

* Add yanix crate

This commit adds `yanix` crate as a Unix dependency for `wasi-common`.
`yanix` stands for Yet Another Nix crate and is exactly what the name
suggests: a crate in the spirit of the `nix` crate, but which takes a different
approach, using lower-level interfaces with less abstraction, so that it fits
better with its main use case, implementation of WASI syscalls.

* Replace nix with yanix crate

Having introduced `yanix` crate as an in-house replacement for the
`nix` crate, this commit makes the necessary changes to `wasi-common`
to depend _only_ on `yanix` crate.

* Address review comments

* make `fd_dup` unsafe
* rename `get_fd` to `get_fd_flags`, etc.
* reuse `io::Error::last_os_error()` to get the last errno value

* Address more comments

* make all `fcntl` fns unsafe
* adjust `wasi-common` impl appropriately

* Make all fns operating on RawFd unsafe

* Fix linux build

* Address more comments

crates/wasi-common/yanix/src/sys/mod.rs

@@ -0,0 +1,27 @@
+use crate::dir::SeekLoc;
+use cfg_if::cfg_if;
+
+cfg_if! {
+    if #[cfg(any(target_os = "linux",
+                 target_os = "android",
+                 target_os = "emscripten"))] {
+        mod linux;
+        pub(crate) use self::linux::*;
+    }
+    else if #[cfg(any(target_os = "macos",
+                      target_os = "ios",
+                      target_os = "freebsd",
+                      target_os = "netbsd",
+                      target_os = "openbsd",
+                      target_os = "dragonfly"))] {
+        mod bsd;
+        pub(crate) use self::bsd::*;
+    } else {
+        compile_error!("yanix doesn't compile for this platform yet");
+    }
+}
+
+pub trait EntryExt {
+    fn ino(&self) -> u64;
+    fn seek_loc(&self) -> SeekLoc;
+}