dillon1000/react

Add `reactive` flag to Place

This is part of a stack for inferring variables which are reactive via *control 
dependencies* as opposed to a data dependency. In compiler engineering, a 
statement S2 is control-dependent on statement S1 if S1 is in the post-dominance 
frontier of S2. Stated more intuitively: if S1 decides whether or not S2 is 
reached, then S1 is a control dependency of S2. 

As a start, we add `Place.reactive: boolean` so that individual places can track 
whether they are reactive or not. This lets us do fine-grained reactivity 
inference on the control-flow graph, even taking into account different SSA 
instances of a variable, so that we can say that a particular SSA version of `x` 
is reactive, while other "versions" of x (due to reassignment) are not.