Normalize index in Vector.at in builtin method

The main culprit of a Vector slowdown (when compared to Array) was the
normalization of the index when accessing the elements.
Turns out that the Graal was very persistent on **not** inlining that
particular fragment and that was degrading the results in benchmarks.

Being unable to force it to do it (looks like a combination of thunk
execution and another layer of indirection) we resorted to just moving
the normalization to the builtin method. That makes Array and Vector
perform roughly the same.

distribution/lib/Standard/Base/0.0.0-dev/src/Data/Vector.enso

@@ -147,8 +147,7 @@ type Vector a
              [1, 2, 3].at -1 == 3
     at : Integer -> Any ! Index_Out_Of_Bounds_Error
     at self index =
-        actual_index = if index < 0 then self.length + index else index
-        Panic.catch Invalid_Array_Index_Error_Data (self.unsafe_at actual_index) _->
+        Panic.catch Invalid_Array_Index_Error_Data (self.at_builtin index) _->
             Error.throw (Index_Out_Of_Bounds_Error_Data index self.length)
 
     ## ADVANCED

engine/runtime/src/main/java/org/enso/interpreter/node/expression/builtin/immutable/AtVectorNode.java

@@ -0,0 +1,33 @@
+package org.enso.interpreter.node.expression.builtin.immutable;
+
+import com.oracle.truffle.api.interop.InteropLibrary;
+import com.oracle.truffle.api.interop.InvalidArrayIndexException;
+import com.oracle.truffle.api.interop.UnsupportedMessageException;
+import com.oracle.truffle.api.nodes.Node;
+import org.enso.interpreter.dsl.BuiltinMethod;
+import org.enso.interpreter.node.expression.builtin.interop.syntax.HostValueToEnsoNode;
+import org.enso.interpreter.runtime.Context;
+import org.enso.interpreter.runtime.data.Vector;
+import org.enso.interpreter.runtime.error.PanicException;
+
+@BuiltinMethod(
+    type = "Vector",
+    name = "at_builtin",
+    description = "Returns an element of Vector at the specified index.")
+public class AtVectorNode extends Node {
+  private @Child InteropLibrary interop = InteropLibrary.getFactory().createDispatched(3);
+  private @Child HostValueToEnsoNode toEnso = HostValueToEnsoNode.build();
+
+  Object execute(Vector self, long index) {
+    try {
+      long actualIndex = index < 0 ? index + self.length(interop) : index;
+      return self.readArrayElement(actualIndex, interop, toEnso);
+    } catch (InvalidArrayIndexException e) {
+      Context ctx = Context.get(this);
+      throw new PanicException(
+          ctx.getBuiltins().error().makeInvalidArrayIndexError(self, index), this);
+    } catch (UnsupportedMessageException e) {
+      throw new PanicException(e.getMessage(), this);
+    }
+  }
+}