utils: add cast_* methods to allow more type-safe casting

Relying on just `as_any` was error prone and will become redundant in
the future, once upcasting will be stable, we also won't to impose a
restriction on to which concrete type we're casting, since casting
to a type that doesn't implement a base trait doesn't make much
sense.

Co-authored-by: Kirill Chibisov <contact@kchibisov.com>

src/platform/x11.rs

@@ -91,7 +91,7 @@ pub trait ActiveEventLoopExtX11 {
 impl ActiveEventLoopExtX11 for dyn ActiveEventLoop + '_ {
     #[inline]
     fn is_x11(&self) -> bool {
-        self.as_any().downcast_ref::<crate::platform_impl::x11::ActiveEventLoop>().is_some()
+        self.cast_ref::<crate::platform_impl::x11::ActiveEventLoop>().is_some()
     }
 }