dillon1000/react

Scaffolding for early return from reactive scopes

Adds a new `earlyReturnValue` property on ReactiveScope which will be set if the 
scope had one or more early returns, with information about the temporary 
identifier that the early return value will be assigned to, as well as the label 
to be used for breaking (to simulate the early-return). The next PR shows the 
intended codegen.