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/helpers.rs

@@ -0,0 +1,20 @@
+use crate::old::snapshot_0::{Error, Result};
+use std::convert::TryInto;
+use std::str;
+use std::time::{SystemTime, UNIX_EPOCH};
+
+pub(crate) fn systemtime_to_timestamp(st: SystemTime) -> Result<u64> {
+    st.duration_since(UNIX_EPOCH)
+        .map_err(|_| Error::EINVAL)? // date earlier than UNIX_EPOCH
+        .as_nanos()
+        .try_into()
+        .map_err(Into::into) // u128 doesn't fit into u64
+}
+
+/// Creates not-owned WASI path from byte slice.
+///
+/// NB WASI spec requires bytes to be valid UTF-8. Otherwise,
+/// `__WASI_ERRNO_ILSEQ` error is returned.
+pub(crate) fn path_from_slice<'a>(s: &'a [u8]) -> Result<&'a str> {
+    str::from_utf8(s).map_err(|_| Error::EILSEQ)
+}