Implement Clone for Value and use it in Value::itself

Avoid calling itself via funcall to work around a stack split that is
documented in GH-145.

The Clone implementation panics if attempting to clone a Value with ruby
type == ruby::Data. We cannot safely increase the ref count on the Rc
smart pointer that backs the void *.

Fixes GH-3.

mruby/src/value/mod.rs

@@ -359,7 +359,7 @@ impl Value {
     where
         T: TryFromMrb<Self, From = types::Ruby, To = types::Rust>,
     {
-        self.funcall::<T, _, _>("itself", &[])
+        self.clone().try_into::<T>()
     }
 
     /// Call `#freeze` on this [`Value`] and consume `self`.
@@ -401,6 +401,18 @@ impl fmt::Debug for Value {
     }
 }
 
+impl Clone for Value {
+    fn clone(&self) -> Self {
+        if self.ruby_type() == types::Ruby::Data {
+            panic!("Cannot safely clone a Value with type tag Ruby::Data.");
+        }
+        Self {
+            interp: self.interp.clone(),
+            value: self.value.clone(),
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use crate::convert::FromMrb;