Add higher order function test cases

valid/function/higher_order_functions/higherManyVars.wacc

+# higher order functions with many variables
+
+# Output:
+# 6
+
+# Exit:
+# 0
+
+# Program:
+
+begin
+  int g(int a, int b, int c) is
+    return a + b + c
+  end
+
+  int{int, int, int} f(int{int, int, int} d) is
+    return d
+  end
+
+  int{int, int, int} x = call f(g);
+  int y = call x(2, 4, 0);
+  println y
+end
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valid/function/higher_order_functions/higherNested.wacc

+# basic higher order function declaration and use
+
+# Output:
+# 2
+
+# Exit:
+# 0
+
+# Program:
+
+begin
+  int g(int a) is
+    return a
+  end
+
+  int{int} f(int{int} d) is
+    return d
+  end
+
+
+  int{int} i(int{int}{int{int}} e, int{int} a) is
+    int{int} k = call e(a);
+    return k
+  end
+
+  int{int} x = call i(f, g);
+  int y = call x(4);
+  println y
+end
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valid/function/higher_order_functions/higherOrderSimple.wacc

+# basic higher order function declaration and use
+
+# Output:
+# 2
+
+# Exit:
+# 0
+
+# Program:
+
+begin
+  int g(int a) is
+    return a
+  end
+
+  int{int} f(int{int} d) is
+    return d
+  end
+
+  int{int} x = call f(g);
+  int y = call x(2);
+  println y
+end
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