Dafny-to-Rust Performance improvement: access sequence by usize, not int

[MikaelMayer]

Consider the following code:

  method First(s: seq<int>) requires |s| != 0 returns (j: int) {
    return s[0]
  }

It will emit the equivalent of

  fn First(s: Sequence<DafnyInt>) -> DafnyInt {
    s.get(int!(0))
  }

which results in a conversion of 0 to DafnyInt and then back to a usize
We could get rid of this conversion issue by emitting

  s.get_usize(truncate!(int!(0), usize))

because in that case, the expression simplifier would just replace it with

  s.get_usize(0)