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)
```
public boolean equals(@NullableDecl Object object) {
    return object == this || delegate().equals(object);
  }
  @Override
  public int hashCode() {
    return delegate().hashCode();
  }
  /**
   * A sensible definition of {@link #putAll(Map)} in terms of {@link #put(Object, Object)}. If you
   * override {@link #put(Object, Object)}, you may wish to override {@link #putAll(Map)} to forward
   * to this implementation.
   *
   * @since 7.0
   */
  protected void standardPutAll(Map<? extends K, ? extends V> map) {
    Maps.putAllImpl(this, map);
  }
  /**
   * A sensible, albeit inefficient, definition of {@link #remove} in terms of the {@code iterator}
   * method of {@link #entrySet}. If you override {@link #entrySet}, you may wish to override {@link
   * #remove} to forward to this implementation.
   *
   * <p>Alternately, you may wish to override {@link #remove} with {@code keySet().remove}, assuming
   * that approach would not lead to an infinite loop.
   *
   * @since 7.0
   */
  @Beta
  protected V standardRemove(@NullableDecl Object key) {
    Iterator<Entry<K, V>> entryIterator = entrySet().iterator();