Add support for wasi_snapshot_preview1. (#592)

* Add support for wasi_snapshot_preview1.

This adds support for the new ABI, while preserving compatibility
support for the old ABI.

* Fix compilation on platforms where nlink_t isn't 64-bit.

* rustfmt

* Fix Windows build errors.

crates/wasi-common/src/old/snapshot_0/sys/unix/mod.rs

@@ -0,0 +1,34 @@
+pub(crate) mod fdentry_impl;
+pub(crate) mod host_impl;
+pub(crate) mod hostcalls_impl;
+
+mod dir;
+mod filetime;
+
+#[cfg(any(
+    target_os = "macos",
+    target_os = "netbsd",
+    target_os = "freebsd",
+    target_os = "openbsd",
+    target_os = "ios",
+    target_os = "dragonfly"
+))]
+mod bsd;
+#[cfg(target_os = "linux")]
+mod linux;
+
+use crate::old::snapshot_0::{Error, Result};
+use std::ffi::CString;
+use std::fs::{File, OpenOptions};
+
+pub(crate) fn dev_null() -> Result<File> {
+    OpenOptions::new()
+        .read(true)
+        .write(true)
+        .open("/dev/null")
+        .map_err(Into::into)
+}
+
+pub(crate) fn str_to_cstring(s: &str) -> Result<CString> {
+    CString::new(s.as_bytes()).map_err(|_| Error::EILSEQ)
+}