Added seq-like API for scroll queries

This is a very early version of it, _please_ don't merge it right away (I bet you won't do that anyway, though.)

Here are problems I see:

1. `scroll-each` shouldn't be in native/document, since it almost doesn't care about API details (however, it needs appropriate doc/scroll)

1. Naming is odd.

1. It doesn't respect scroll time passed to the initializing search.

1. I have no idea how to use function composition and other fancy functional programming techniques.

Would love to hear any thoughts on how to make this better.

Thanks!