Introduce MaybeSend trait in iced_futures

It allows to clean up all the `trait_aliases` modules!

futures/src/maybe_send.rs

@@ -0,0 +1,21 @@
+#[cfg(not(target_arch = "wasm32"))]
+mod platform {
+    /// An extension trait that enforces `Send` only on native platforms.
+    ///
+    /// Useful to write cross-platform async code!
+    pub trait MaybeSend: Send {}
+
+    impl<T> MaybeSend for T where T: Send {}
+}
+
+#[cfg(target_arch = "wasm32")]
+mod platform {
+    /// An extension trait that enforces `Send` only on native platforms.
+    ///
+    /// Useful to write cross-platform async code!
+    pub trait MaybeSend {}
+
+    impl<T> MaybeSend for T {}
+}
+
+pub use platform::MaybeSend;