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/clock.rs

@@ -0,0 +1,29 @@
+use crate::{Errno, Result};
+use std::mem::MaybeUninit;
+
+#[derive(Debug, Copy, Clone)]
+pub enum ClockId {
+    Realtime,
+    Monotonic,
+    ProcessCPUTime,
+    ThreadCPUTime,
+}
+
+impl ClockId {
+    pub fn as_raw(&self) -> libc::clockid_t {
+        match self {
+            Self::Realtime => libc::CLOCK_REALTIME,
+            Self::Monotonic => libc::CLOCK_MONOTONIC,
+            Self::ProcessCPUTime => libc::CLOCK_PROCESS_CPUTIME_ID,
+            Self::ThreadCPUTime => libc::CLOCK_THREAD_CPUTIME_ID,
+        }
+    }
+}
+
+pub fn clock_getres(clock_id: ClockId) -> Result<libc::timespec> {
+    let mut timespec = MaybeUninit::<libc::timespec>::uninit();
+    Errno::from_success_code(unsafe {
+        libc::clock_getres(clock_id.as_raw(), timespec.as_mut_ptr())
+    })?;
+    Ok(unsafe { timespec.assume_init() })
+}