Fills all keys contained in the receiver into the specified list. Fills the list, starting at index 0. After this call returns the specified list has a new size that equals
this.size(). Iteration order is guaranteed to be
identical to the order used by method {@link #forEachKey(IntProcedure)}.
This method can be used to iterate over the keys of the receiver.
@param list the list to be filled, can have any size.