remove a few warnings

src/main/java/carpet/script/exception/CarpetExpressionException.java

@@ -8,7 +8,7 @@ import java.util.List;
 
 public class CarpetExpressionException extends RuntimeException
 {
-    public List<FunctionValue> stack;
+    public final List<FunctionValue> stack;
 
     public CarpetExpressionException(String message, List<FunctionValue> stack)
     {

src/main/java/carpet/script/exception/ExitStatement.java

@@ -5,7 +5,7 @@ import carpet.script.value.Value;
 /* Exception thrown to terminate execution mid expression (aka return statement) */
 public class ExitStatement extends RuntimeException
 {
-    public Value retval;
+    public final Value retval;
     public ExitStatement(Value value)
     {
         retval = value;

src/main/java/carpet/script/exception/ExpressionException.java

@@ -15,8 +15,8 @@ import static carpet.script.ExpressionInspector.Expression_getName;
 /* The expression evaluators exception class. */
 public class ExpressionException extends RuntimeException
 {
-    public Context context;
-    public List<FunctionValue> stack = new ArrayList<>();
+    public final Context context;
+    public final List<FunctionValue> stack = new ArrayList<>();
     public ExpressionException(Context c, String message)
     {
         super(makeMessage(c, null, null, message));
@@ -35,7 +35,7 @@ public class ExpressionException extends RuntimeException
         context = c;
     }
 
-    private static Fluff.TriFunction<Expression, Tokenizer.Token, String, List<String>> errorMaker = (expr, token, errmessage) ->
+    private static final Fluff.TriFunction<Expression, Tokenizer.Token, String, List<String>> errorMaker = (expr, token, errmessage) ->
     {
 
         List<String> errMsg = new ArrayList<>();