Vectors are dense.

Removes the concept of sparcity from Vector, causing unset index and index set to the value `undefined` to be indistinguishable.

See #123 for context on this decision.

The resulting behavioral changes from this diff:

 * Vector iterations now include all indicies 0 .. length
 * Vector look-up no longer checks hasOwnProperty
 * Vector has(n) is now a cheap comparison to length.
 * fromEntriesSequence now skips over nullish values in the parent iterator.
Ensure Vector iteration behaves as if Vector were dense
ArraySequence is iterated densely and treats has as a cheap operation. take and takeWhile no longer need custom overrides.
Simplify `join` to not deal with sparse sequences
No longer need an override of valueSeq since indexed sequences are dense
note