Use rsix to make system calls in Wasmtime. (#3355)

* Use rsix to make system calls in Wasmtime.

`rsix` is a system call wrapper crate that we use in `wasi-common`,
which can provide the following advantages in the rest of Wasmtime:

 - It eliminates some `unsafe` blocks in Wasmtime's code. There's
   still an `unsafe` block in the library, but this way, the `unsafe`
   is factored out and clearly scoped.

 - And, it makes error handling more consistent, factoring out code for
   checking return values and `io::Error::last_os_error()`, and code that
   does `errno::set_errno(0)`.

This doesn't cover *all* system calls; `rsix` doesn't implement
signal-handling APIs, and this doesn't cover calls made through `std` or
crates like `userfaultfd`, `rand`, and `region`.

crates/runtime/Cargo.toml

@@ -29,6 +29,9 @@ anyhow = "1.0.38"
 [target.'cfg(target_os = "macos")'.dependencies]
 mach = "0.3.2"
 
+[target.'cfg(unix)'.dependencies]
+rsix = "0.23.2"
+
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = { version = "0.3.7", features = ["winbase", "memoryapi", "errhandlingapi", "handleapi"] }