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)
```
protected Collection<Range<C>> delegate() {
      return delegate;
    }
    @Override
    public int hashCode() {
      return Sets.hashCodeImpl(this);
    }
    @Override
    public boolean equals(@Nullable Object o) {
      return Sets.equalsImpl(this, o);
    }
  }
  @Override
  public @Nullable Range<C> rangeContaining(C value) {
    checkNotNull(value);
    Entry<Cut<C>, Range<C>> floorEntry = rangesByLowerBound.floorEntry(Cut.belowValue(value));
    if (floorEntry != null && floorEntry.getValue().contains(value)) {
      return floorEntry.getValue();
    } else {
      
      return null;
    }
  }
  @Override
  public boolean intersects(Range<C> range) {
    checkNotNull(range);
    Entry<Cut<C>, Range<C>> ceilingEntry = rangesByLowerBound.ceilingEntry(range.lowerBound);
    if (ceilingEntry != null
        && ceilingEntry.getValue().isConnected(range)
        && !ceilingEntry.getValue().intersection(range).isEmpty()) {