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)
```
}
    @Override
    public boolean isCancelled() {
      throw new AssertionFailedError();
    }
    @Override
    public boolean isDone() {
      /*
       * If isDone is true during the call to listenInPoolThread,
       * listenInPoolThread doesn't start a thread. Make sure it's false the
       * first time through (and forever after, since no one else cares about
       * it).
       */
      return false;
    }
  }
  private static final class RecordingRunnable implements Runnable {
    final CountDownLatch wasRun = new CountDownLatch(1);
    
    @Override
    public synchronized void run() {
      checkState(wasRun.getCount() > 0);
      wasRun.countDown();
    }
  }
  @SuppressWarnings("IsInstanceIncompatibleType") 
  public void testListenInPoolThreadRunsListenerAfterRuntimeException() throws Exception {
    RuntimeExceptionThrowingFuture<String> input = new RuntimeExceptionThrowingFuture<>();
    /*
     * The compiler recognizes that "input instanceof ListenableFuture" is