dillon1000/react

Initial (flagged) support for reactive scopes with early return

Adds support for early returns within reactive scopes, behind a new feature 
flag. The flag is off by default, where this case continues to throw a Todo 
bailout. 

Since implementing a sketch of the codegen in the previous PR I realized that 
it's easy enough to implement the more optimal output, so i've updated that 
here. Rather than both the if and else branch of the reactive scope having an 
"if the return value was not a sentinel return it" check, we instead make the 
return temporary a proper declaration of the reactive scope. Then, since it's 
actually an output it's available in the outer block scope, and we could do a 
single if-return after the reactive scope. 

Edit: see comment below for thoughts on test cases.