ImmutableSortedMap collectors that do not require a Comparator for Comparable keys

[pianojosh]

Essentially syntactic sugar to provide parity to the other immutable map collectors when possible.

I believe this can be added relatively simply in https://github.com/google/guava/blob/master/guava/src/com/google/common/collect/ImmutableSortedMap.java:

  public static <T, K extends Comparable<K>, V> Collector<T, ?, ImmutableSortedMap<K, V>> toImmutableSortedMap(
      Function<? super T, ? extends K> keyFunction,
      Function<? super T, ? extends V> valueFunction) {
    return toImmutableSortedMap(Comparable::compareTo, keyFunction, valueFunction);
  }

And:

  public static <T, K extends Comparable<K>, V> Collector<T, ?, ImmutableSortedMap<K, V>> toImmutableSortedMap(
      Function<? super T, ? extends K> keyFunction,
      Function<? super T, ? extends V> valueFunction,
      BinaryOperator<V> mergeFunction) {
    return toImmutableSortedMap(Comparable::compareTo, keyFunction, valueFunction, mergeFunction);
  }