dillon1000/react

Infer sets of variables in each reactive scope

Adds a new pass `InferReactiveScopeVariables` which determines the sets of 
variables (by Identifier) which "construct together" and belong in the same 
reactive scope. Concretely, `Identifier` gets a new property `scope: ScopeId`, 
and this pass assigns each identifier a ScopeId value. The algorithm iterates 
over all instructions in all blocks (in a single pass) and builds up disjoint 
sets of identifiers that appear as mutable operands in the same instruction. 

The algorithm is relatively simple (especially since I had already implemented a 
union-find data structure): however looking at some examples reinforced that 
other planned todos around alias analysis are really important. We also have to 
think more about what "mutable lifetime" means in the context of SSA: currently 
variables that are reassigned (but never "mutated", eg bc they're assigned a 
value type) never appear as mutable.