rename `bindToCurrentDataIfIsfunction` to `bindDataContext`

(for clarity)
Reorg observe-sequence

(in preparation for {{#each}} over objects)
Eliminate `_fetch` on handles returned from `cursor.observe()`

This was originally introduced with
f1b77fec966c8c97d53628bfd79196afb0c99498, but it looks
like all of our tests now pass. (Maybe eliminating `rewind` in
b5a0613f85002ee35732c01a27c0d9b1901ed0de made this no
longer necessary?)

If we find that this commit did break something, let's make sure to
add a failing test before reverting.