Implement fd_filestat_get for all platforms (#42)

* Implement fd_filestat_get for all platforms

* Remove an old comment

* Remove panics from the syscall wrappers

* Return WASI error type

* Reuse Metadata if possible to save syscalls.

* Refactor the change for two separate fd_filestat_get_impl

* Refactor error handling

src/helpers.rs

@@ -0,0 +1,10 @@
+use crate::{host, Result};
+use std::convert::TryInto;
+use std::time::{SystemTime, UNIX_EPOCH};
+pub(crate) fn systemtime_to_timestamp(st: SystemTime) -> Result<u64> {
+    st.duration_since(UNIX_EPOCH)
+        .map_err(|_| host::__WASI_EINVAL)? // date earlier than UNIX_EPOCH
+        .as_nanos()
+        .try_into()
+        .map_err(|_| host::__WASI_EOVERFLOW) // u128 doesn't fit into u64
+}