Use the libstd instead of the errno crate in wasmtime-runtime. (#408)

Rust's standard library now has a way to read the OS errno value, so use
that instead of depending on the errno crate in wasmtime-runtime.

Cargo.toml

@@ -35,7 +35,6 @@ pretty_env_logger = "0.3.0"
 file-per-thread-logger = "0.1.1"
 wabt = "0.9.2"
 libc = "0.2.60"
-errno = "0.2.4"
 rayon = "1.1"
 wasm-webidl-bindings = "0.5"