wasi-common error cleanup: part 1, yanix (#1226)

* Reuse std::io::Error for raw *nix errno

This commit removes custom `yanix::Errno` and instead (as was
previously suggested) reuses `std::io::Error` to generate and wrap
raw *nix errno value.

* Update wasi-common to use new Yanix error type

This commit updates `wasi-common` to use new way of handling raw
OS error in `yanix`; i.e., via re-use of `std::io::Error` instead
of a custom `Errno` enum.

* Fix formatting

* Unwrap if io::Error created from raw OS error

This commit calls `unwrap` on `err` if that one was created via
`io::Error::last_os_error()`. It also refactors error matching
in several syscalls on the BSD platform (mainly).

crates/wasi-common/src/error.rs

@@ -101,9 +101,6 @@ pub enum Error {
     Wasi(#[from] WasiError),
     #[error("IO error: {0}")]
     Io(#[from] std::io::Error),
-    #[cfg(unix)]
-    #[error("Yanix error: {0}")]
-    Yanix(#[from] yanix::YanixError),
 }
 
 impl From<TryFromIntError> for Error {
@@ -150,16 +147,6 @@ impl Error {
                 };
                 err.as_wasi_error()
             }
-            #[cfg(unix)]
-            Self::Yanix(err) => {
-                use yanix::YanixError::*;
-                let err: Self = match err {
-                    Errno(errno) => (*errno).into(),
-                    NulError(err) => err.into(),
-                    TryFromIntError(err) => (*err).into(),
-                };
-                err.as_wasi_error()
-            }
         }
     }