Remove transactions for now -- it's basically just stream.tick() now that does the tricks.
Reorganize, remove old code.
Remove the last references to transactions.
Simplificate.
Documentation. And preparing to rename update -> set (again!) and implement a new thing, 'refresh', which will be renamed to update.
Rename Stream.update() to Stream.set().
Make update() set() and refresh() update().

This broke something in playground/cells.html, TODO take a look at it.
Make .zip shorter.
Implement .update() ('pulling') quite inelegantly.  Now all updaters must use valueOf(parent) to get the parent's value.
Start to reorganize tests.
Remove old code.