Enable instant only for wasm32 targets

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

native/src/time.rs

@@ -0,0 +1,7 @@
+//! Keep track of time, both in native and web platforms!
+
+#[cfg(target_arch = "wasm32")]
+pub use instant::{Duration, Instant};
+
+#[cfg(not(target_arch = "wasm32"))]
+pub use std::time::{Duration, Instant};