Unable to modify subRangeSet returned by TreeRangeSet.subRangeSet(Range).
IMO, it should be availiable to do so as specified in the document.

```java
import com.google.common.collect.Range;
import com.google.common.collect.TreeRangeSet;
import org.junit.jupiter.api.Test;

@SuppressWarnings("UnstableApiUsage")
public class SubRangeSetTest {

    @Test
    public void testAdd() {
        TreeRangeSet<Integer> set = TreeRangeSet.create();
        Range<Integer> range = Range.closedOpen(0, 5);
        set.subRangeSet(range).add(range);
    }

    @Test
    public void testReplaceAdd() {
        TreeRangeSet<Integer> set = TreeRangeSet.create();
        Range<Integer> range = Range.closedOpen(0, 5);
        set.add(range);
        set.subRangeSet(range).add(range);
    }
}
```

```
java.lang.UnsupportedOperationException
	at java.base/java.util.AbstractMap.put(AbstractMap.java:209)
	at com.google.common.collect.TreeRangeSet.replaceRangeWithSameLowerBound(TreeRangeSet.java:266)
	at com.google.common.collect.TreeRangeSet.add(TreeRangeSet.java:216)
	at com.google.common.collect.TreeRangeSet$SubRangeSet.add(TreeRangeSet.java:894)
	at SubRangeSetTest.testAdd(SubRangeSetTest.java:12)
```
```
java.lang.UnsupportedOperationException
	at com.google.common.collect.UnmodifiableIterator.remove(UnmodifiableIterator.java:46)
	at com.google.common.collect.Iterators.clear(Iterators.java:985)
	at com.google.common.collect.Maps$IteratorBasedAbstractMap.clear(Maps.java:3562)
	at com.google.common.collect.TreeRangeSet.add(TreeRangeSet.java:214)
	at com.google.common.collect.TreeRangeSet$SubRangeSet.add(TreeRangeSet.java:894)
	at SubRangeSetTest.testReplaceAdd(SubRangeSetTest.java:20)
```
&& upperBoundWindow.upperBound.isLessThan(backingItr.peek().upperBound)) {
        backingItr.next();
      }
      return new AbstractIterator<Entry<Cut<C>, Range<C>>>() {
        @Override
        protected Entry<Cut<C>, Range<C>> computeNext() {
          if (!backingItr.hasNext()) {
            return endOfData();
          }
          Range<C> range = backingItr.next();
          return upperBoundWindow.lowerBound.isLessThan(range.upperBound)
              ? Maps.immutableEntry(range.upperBound, range)
              : endOfData();
        }
      };
    }
    @Override
    public int size() {
      if (upperBoundWindow.equals(Range.all())) {
        return rangesByLowerBound.size();
      }
      return Iterators.size(entryIterator());
    }
    @Override
    public boolean isEmpty() {
      return upperBoundWindow.equals(Range.all())
          ? rangesByLowerBound.isEmpty()
          : !entryIterator().hasNext();
    }
  }