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/test-programs/Cargo.toml

@@ -21,7 +21,7 @@ tempfile = "3.1.0"
 os_pipe = "0.9"
 anyhow = "1.0.19"
 wat = "1.0.37"
-cap-std = "0.19.0"
+cap-std = "0.19.1"
 tokio = { version = "1.8.0", features = ["rt-multi-thread"] }
 
 [features]