Fix timestamp-to-nanosecond conversion in the adapter (bytecodeallian…

…ce#93)

The order of operations for handling the seconds/nanoseconds part of the
structure were a bit off which meant that a nonzero nanosecond field
would have an unexpected effect on the result.

Closes bytecodealliance#92

host/tests/runtime.rs

@@ -132,7 +132,7 @@ async fn run_time(mut store: Store<WasiCtx>, wasi: WasiCommand) -> Result<()> {
         }
 
         fn now(&self) -> Duration {
-            Duration::from_secs(1431648000)
+            Duration::new(1431648000, 100)
         }
 
         fn dup(&self) -> Box<dyn WasiWallClock + Send + Sync> {

test-programs/src/bin/time.rs

@@ -6,7 +6,7 @@ fn main() {
     assert_eq!(Duration::from_secs(42), then.elapsed());
 
     assert_eq!(
-        SystemTime::UNIX_EPOCH + Duration::from_secs(1431648000),
+        SystemTime::UNIX_EPOCH + Duration::new(1431648000, 100),
         SystemTime::now()
     );
 }