String builder

[amitu]

We have a decent dim builder right now. We also have markup support for building dom. How do we build more complex strings? Markup support can be extended to strings.

We also want to do "container management for strings", say a string contains a slot, and in that slot we want to put some strings. Basically we want to make some slot "current" so future string operations apply on that current slot.

We can have push operation which works on current slot:

-- string foo:

Hello, {slot-id}!

-- slot: slot-id

-- push: yo!
if: $x

-- push: $o.name
$loop$: $objects as $o

We will have to do all this on frontend as well.