replace field string list with SelectableExpression list in SelectExpression

src/main/java/jef/expressions/IntermediateFieldExpression.java

@@ -0,0 +1,26 @@
+package jef.expressions;
+
+import jef.expressions.selectable.SelectableExpression;
+import lombok.Getter;
+
+@Getter
+public class IntermediateFieldExpression extends ConstantExpression implements SelectableExpression, Expression {
+    private final String name;
+    private final String classDescriptor;
+
+    public IntermediateFieldExpression(String name, String classDescriptor) {
+        super(name);
+        this.name = name;
+        this.classDescriptor = classDescriptor;
+    }
+
+    @Override
+    public Type getType() {
+        return Type.INTERMEDIATE_FIELD;
+    }
+
+    @Override
+    public String toString() {
+        return name;
+    }
+}