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/src/sys/unix/mod.rs

@@ -2,7 +2,6 @@ pub(crate) mod fdentry_impl;
 pub(crate) mod host_impl;
 pub(crate) mod hostcalls_impl;
 
-mod dir;
 mod filetime;
 
 #[cfg(any(
@@ -17,8 +16,7 @@ mod bsd;
 #[cfg(target_os = "linux")]
 mod linux;
 
-use crate::{Error, Result};
-use std::ffi::CString;
+use crate::Result;
 use std::fs::{File, OpenOptions};
 use std::path::Path;
 
@@ -30,10 +28,6 @@ pub(crate) fn dev_null() -> Result<File> {
         .map_err(Into::into)
 }
 
-pub(crate) fn str_to_cstring(s: &str) -> Result<CString> {
-    CString::new(s.as_bytes()).map_err(|_| Error::EILSEQ)
-}
-
 pub fn preopen_dir<P: AsRef<Path>>(path: P) -> Result<File> {
     File::open(path).map_err(Into::into)
 }