Replace parentTdBody, tdBody, indexPath with a FrameStack

[jimmyhchan]

• introduce a framestack object which keeps track of tdBody, elIndex/count and placeholders
• have the instructions increment the framestack values to represent where we are within tornado/elements/placeholders
• remove (now) unused code from context

the value of the framestack inside of instructions being [inner, outer] might make more sense as [before, after] this might change when we eventually rejigger the generateJS step.

Next steps

• blockIndex can prolly move into this as well
• Not sure if It make sense that attributes create a placeholder seems we're doing some tricky stuff with hasTornadoRef

[smfoote]

This looks really good. I would love to see blockIndex pulled out as well, and made somehow extensible. As far as I can tell, that is the last piece before blocks and inline-partials can become true extensions.