Support 4-byte unicode escape sequences in string literals.

Author: fangerer

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/parser/PythonTreeTranslator.java

@@ -730,6 +730,17 @@ private static String unescapeJavaString(String st) {
                         sb.append(Character.toChars(code));
                         i += 5;
                         continue;
+                    // Hex Unicode: U????????
+                    case 'U':
+                        if (i >= st.length() - 9) {
+                            ch = 'U';
+                            break;
+                        }
+                        code = Integer.parseInt(st.substring(i + 2, i + 10), 16);
+                        sb.append(Character.toChars(code));
+                        i += 9;
+                        continue;
+                    // Hex Unicode: x??
                     case 'x':
                         if (i >= st.length() - 3) {
                             ch = 'u';