Enable instant only for wasm32 targets

... and hide the dependency under a `time` module in `iced_native`

src/time.rs

@@ -1,12 +1,12 @@
 //! Listen and react to time.
+pub use crate::runtime::time::{Duration, Instant};
+
 use crate::Subscription;
 
 /// Returns a [`Subscription`] that produces messages at a set interval.
 ///
 /// The first message is produced after a `duration`, and then continues to
 /// produce more messages every `duration` after that.
-pub fn every(
-    duration: std::time::Duration,
-) -> Subscription<std::time::Instant> {
+pub fn every(duration: Duration) -> Subscription<Instant> {
     iced_futures::time::every(duration)
 }