le will be advanced and its items will be cached until
they are used by the child iterables. This may require significant storage.

By default, attempting to select a bucket to which no items belong  will
exhaust the iterable and cache all values.
If you specify a *validator* function, selected buckets will instead be
checked against it.

    >>> from itertools import count
    >>> it = count(1, 2)  # Infinite sequence of odd numbers
    >>> key = lambda x: x % 10  # Bucket by last digit
    >>> validator = lambda x: x in {1, 3, 5, 7, 9}  # Odd digits only
    >>> s = bucket(it, key=key, validator=validator)
    >>> 2 in s
    False
    >>> list(s[2])
    []

Nc