track last-seen occurrence of Vars separately

[gridaphobe]

given a program

  let x = 0
  if cond {
    let x = 1
  } else {
    let x = x + 1
  }

we want to ensure that the RHS occurrence of x in the else-branch refers to the x outside of the if, not the x in the then-branch.

With a single copy of the symEnv we would introduce a new binder for x in the then-branch and then see refer to that binder in the else-branch, which is bogus. So, instead we keep a copy of the env that is used to lookup the last-seen instance of a Var, which we can reset after running a branch.

[gridaphobe]

This fixes #69, though it might be simpler to just use a global supply of fresh integers, rather than trying to track the count for each Var individually 😄

[leepike]

Closing this old pull request...