Replaced impl_cast with a new and improved trait

With this trait the impl_cast macros are no longer needed, simplifying
everything.
This commit also improved how the actual casting itself is handled.

automation_cast/src/lib.rs

@@ -0,0 +1,37 @@
+#![allow(incomplete_features)]
+#![feature(specialization)]
+#![feature(unsize)]
+
+use std::marker::Unsize;
+
+pub trait Cast<P: ?Sized> {
+    fn cast(&self) -> Option<&P>;
+    fn cast_mut(&mut self) -> Option<&mut P>;
+}
+
+impl<D, P> Cast<P> for D
+where
+    P: ?Sized,
+{
+    default fn cast(&self) -> Option<&P> {
+        None
+    }
+
+    default fn cast_mut(&mut self) -> Option<&mut P> {
+        None
+    }
+}
+
+impl<D, P> Cast<P> for D
+where
+    D: Unsize<P>,
+    P: ?Sized,
+{
+    fn cast(&self) -> Option<&P> {
+        Some(self)
+    }
+
+    fn cast_mut(&mut self) -> Option<&mut P> {
+        Some(self)
+    }
+}