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/host.rs

@@ -0,0 +1,109 @@
+//! WASI host types. These are types that contain raw pointers and `usize`
+//! values, and so are platform-specific.
+
+#![allow(non_camel_case_types)]
+#![allow(non_snake_case)]
+
+use crate::old::snapshot_0::wasi::*;
+use std::{io, slice};
+use wig::witx_host_types;
+
+witx_host_types!("old/snapshot_0" "wasi_unstable");
+
+pub(crate) unsafe fn ciovec_to_host(ciovec: &__wasi_ciovec_t) -> io::IoSlice {
+    let slice = slice::from_raw_parts(ciovec.buf as *const u8, ciovec.buf_len);
+    io::IoSlice::new(slice)
+}
+
+pub(crate) unsafe fn iovec_to_host_mut(iovec: &mut __wasi_iovec_t) -> io::IoSliceMut {
+    let slice = slice::from_raw_parts_mut(iovec.buf as *mut u8, iovec.buf_len);
+    io::IoSliceMut::new(slice)
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    #[test]
+    fn bindgen_test_layout___wasi_prestat_t() {
+        assert_eq!(
+            ::std::mem::size_of::<__wasi_prestat_t>(),
+            16usize,
+            concat!("Size of: ", stringify!(__wasi_prestat_t))
+        );
+        assert_eq!(
+            ::std::mem::align_of::<__wasi_prestat_t>(),
+            8usize,
+            concat!("Alignment of ", stringify!(__wasi_prestat_t))
+        );
+        assert_eq!(
+            unsafe { &(*(::std::ptr::null::<__wasi_prestat_t>())).pr_type as *const _ as usize },
+            0usize,
+            concat!(
+                "Offset of field: ",
+                stringify!(__wasi_prestat_t),
+                "::",
+                stringify!(pr_type)
+            )
+        );
+        assert_eq!(
+            unsafe { &(*(::std::ptr::null::<__wasi_prestat_t>())).u as *const _ as usize },
+            8usize,
+            concat!(
+                "Offset of field: ",
+                stringify!(__wasi_prestat_t),
+                "::",
+                stringify!(u)
+            )
+        );
+    }
+
+    #[test]
+    fn bindgen_test_layout___wasi_prestat_t___wasi_prestat_u___wasi_prestat_u_dir_t() {
+        assert_eq!(
+            ::std::mem::size_of::<__wasi_prestat_dir_t>(),
+            8usize,
+            concat!("Size of: ", stringify!(__wasi_prestat_dir_t))
+        );
+        assert_eq!(
+            ::std::mem::align_of::<__wasi_prestat_dir_t>(),
+            8usize,
+            concat!("Alignment of ", stringify!(__wasi_prestat_dir_t))
+        );
+        assert_eq!(
+            unsafe {
+                &(*(::std::ptr::null::<__wasi_prestat_dir_t>())).pr_name_len as *const _ as usize
+            },
+            0usize,
+            concat!(
+                "Offset of field: ",
+                stringify!(__wasi_prestat_dir_t),
+                "::",
+                stringify!(pr_name_len)
+            )
+        );
+    }
+
+    #[test]
+    fn bindgen_test_layout___wasi_prestat_t___wasi_prestat_u() {
+        assert_eq!(
+            ::std::mem::size_of::<__wasi_prestat_u_t>(),
+            8usize,
+            concat!("Size of: ", stringify!(__wasi_prestat_u_t))
+        );
+        assert_eq!(
+            ::std::mem::align_of::<__wasi_prestat_u_t>(),
+            8usize,
+            concat!("Alignment of ", stringify!(__wasi_prestat_u_t))
+        );
+        assert_eq!(
+            unsafe { &(*(::std::ptr::null::<__wasi_prestat_u_t>())).dir as *const _ as usize },
+            0usize,
+            concat!(
+                "Offset of field: ",
+                stringify!(__wasi_prestat_u_t),
+                "::",
+                stringify!(dir)
+            )
+        );
+    }
+}