Add a simple test for exitential type

Author: topecongiro

tests/source/existential_type.rs

@@ -0,0 +1,11 @@
+// Existential type.
+
+    #![feature(existential_type)]
+
+pub existential type Adder<F, T>
+where
+    T: Clone,
+    F: Copy
+    : Fn(T) -> T;
+
+pub existential type Adderrr<T>: Fn(  T  ) -> T;

tests/target/existential_type.rs

@@ -0,0 +1,11 @@
+// Existential type.
+
+#![feature(existential_type)]
+
+pub existential type Adder<F, T>
+where
+    T: Clone,
+    F: Copy,
+: Fn(T) -> T;
+
+pub existential type Adderrr<T>: Fn(T) -> T;