Support amortized traversal in forward seeking.

``Iterator`` has been updated to include ``Iterator.Seek``, which
follows the same protocol of ``SkipList.Seek``.  This new ``Seek``
call allows users of goskiplist to amortize traversal costs in batched
forward scan operations.  If the requested key occurs before the
current seek point's key, ``SkipList`` resumes the search from the head
of the list.

An overview of the performance characteristics are provided:

mtp:skiplist mtp$ go test -test.bench="." ./...
PASS
BenchmarkLookup16	 5000000	       243 ns/op
BenchmarkLookup256	 5000000	       556 ns/op
BenchmarkLookup65536	 1000000	      2239 ns/op
BenchmarkSet16	 1000000	      5305 ns/op
BenchmarkSet256	 1000000	      6356 ns/op
BenchmarkSet65536	 1000000	      5073 ns/op
BenchmarkRandomSeek	 1000000	      5105 ns/op
BenchmarkForwardSeek	 1000000	      2975 ns/op
BenchmarkForwardSeekReusedIterator	 1000000	1881 ns/op
ok  	github.com/ryszard/goskiplist/skiplist	96.178s

Compare ``BenchmarkForwardSeek`` versus
``BenchmarkForwardSeekReusedIterator``.

I have a use case where this type of seeking performance will be
tremendously beneficial in reducing operation latency.