dillon1000/react

[valueblocks] More general visitor for ReactiveFunctions

This adds a truly general-purpose visitor pattern for ReactiveFunction, modeled 
on what's worked well in Relay Compiler. All types of node that have children 
get a visitFoo/traverseFoo pair of functions. By default the visitFoo() function 
delegates to the traverseFoo() function, but the visit variant is meant to be 
overridden and can delegate to the traverseFoo() variant — this gives you 
precise control so that you can save/restore state before/after traversing 
children. 

Probably the only interesting thing is that visitLValue() does not call 
visitPlace() by default though it technically could. So far that is making sense 
in the passes i converted. 

Note that once all passes are updated to use this, i'll delete the other visitor 
helpers for ReactiveFunction.