public Item withSignature(CstString newSignature) {
      return new Item(startPc, length, name, descriptor, newSignature, index);
    }
Constructs and returns an instance which is identical to this one, except that the signature
is changed to the given value.

@param newSignature {@code non-null;} the new signature
@return {@code non-null;} an appropriately-constructed instance
android/android_framework/dex-maker/src/main/java/com/android/dx/cf/code/LocalVariableList.java