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/lib.rs

@@ -0,0 +1,40 @@
+//! `yanix` stands for Yet Another Nix crate, and, well, it is simply
+//! a yet another crate in the spirit of the [nix] crate. As such,
+//! this crate is inspired by the original `nix` crate, however,
+//! it takes a different approach, using lower-level interfaces with
+//! less abstraction, so that it fits better with its main use case
+//! which is our WASI implementation, [wasi-common].
+//!
+//! [nix]: https://github.com/nix-rust/nix
+//! [wasi-common]: https://github.com/bytecodealliance/wasmtime/tree/master/crates/wasi-common
+#![cfg(unix)]
+
+pub mod clock;
+pub mod dir;
+pub mod fcntl;
+pub mod file;
+pub mod poll;
+pub mod socket;
+
+mod errno;
+mod sys;
+
+pub mod fadvise {
+    pub use super::sys::fadvise::*;
+}
+
+pub use errno::Errno;
+use std::{ffi, num};
+use thiserror::Error;
+
+pub type Result<T> = std::result::Result<T, YanixError>;
+
+#[derive(Debug, Error)]
+pub enum YanixError {
+    #[error("raw os error {0}")]
+    Errno(#[from] Errno),
+    #[error("a nul byte was not found in the expected position")]
+    NulError(#[from] ffi::NulError),
+    #[error("integral type conversion failed")]
+    TryFromIntError(#[from] num::TryFromIntError),
+}