dillon1000/react

Rename variables during BuildHIR

Per design discussion, this PR changes BuildHIR to maintain the invariant that, 
for each distinct variable in the input, that all references to that variable in 
the HIR will have the same unique `name` _and_ same unique `id`. Phrased 
differently: Identifiers with the same id will have the same name and 
vice-versa. 

This isn't an invariant we maintain throughout compilation — SSA form changes 
the `id`s — but crucially, ensuring that the `name` is also unique allows us to 
understand later which identifiers referred to the same original variable and 
which were different. 

Follow-up PRs will ensure that we maintain variable identifiers in the output as 
well, in all cases except shadowing (and for shadowing, we'll rewrite 
identifiers inside lambdas).