Redo the visitor so that we can target children better

[jimmyhchan]

This adds a buildStackItem pass to mimic what we were doing with Context and Instructions. That pass should be temporary and should be replaced with some better APIs that only add the context that is necessary (TBD).

For examples:

• remove tdIndex from buildStackItem and do a pass that adds index number instead
• replace the namespace logic to something else
• replace escaplable Raw
• move the instruction logic into something else?

[smfoote]

Looks good to me. Do the tests pass? Has @prashn64 taken a look yet?

[jimmyhchan]

yes. tests's pass. @prashn64 has not looked at it?