Stack is a thread-safe synchronized collection. (#696)

When thread safety is not required it's better to use non-thread safe alternative ArrayDeque

terminal/src/main/java/org/jline/utils/Curses.java

@@ -12,7 +12,7 @@
 import java.io.IOError;
 import java.io.IOException;
 import java.io.StringWriter;
-import java.util.Stack;
+import java.util.ArrayDeque;
 
 /**
  * Curses helper methods.
@@ -21,8 +21,8 @@
  */
 public final class Curses {
 
-    private static Object[] sv = new Object[26];
-    private static Object[] dv = new Object[26];
+    private static final Object[] sv = new Object[26];
+    private static final Object[] dv = new Object[26];
 
     private static final int IFTE_NONE = 0;
     private static final int IFTE_IF = 1;
@@ -68,7 +68,7 @@ private static void doTputs(Appendable out, String str, Object... params) throws
         int length = str.length();
         int ifte = IFTE_NONE;
         boolean exec = true;
-        Stack<Object> stack = new Stack<>();
+        ArrayDeque<Object> stack = new ArrayDeque<>();
         while (index < length) {
             char ch = str.charAt(index++);
             switch (ch) {