optimize locals to the stack

[disruptek]

When locals aren't used in remote legs, we don't need to move them to the environment.

[alaviss]

This one is particularly hard, consider:

proc foo() =
  var s = "string"
  var c = cstring(s)
  noop()
  echo $c

Despite only c is used in the following continuation, we must keep s because c leases s resource.