Nested scope closure

[efaust]

There are two bugs here:

First, catch blocks had mismatched stack pushes and pops, leading to total confusion.
Second, we were blurring all seen names into flat levels, meaning that we couldn't tell sibling uses and bindings from nested uses and bindings.

Previously,

var a;
(function () {
    a;
    {
        let a;
    }
})

did not mark the var as closed over, even though it clearly is, since it saw the sibling binding.

[Yoric]

Nit:

if let Some(mut parent_free) = self.free_names-in_block_stack.last_mut() {
  // ...
}

and get rid of unwrapped.

[Yoric]

Nit: Here, you should inspect a value, rather than create a new one for comparison purposes.

if let Some(&true) = unwrapped.get(&name) {
  // ...
}

Except in this case, it looks like you want to update the entry, so it's rather something along the lines of

let new_is_cross_function = old_cross_function || function_scope;

unwrapped.entry(&name)
  // Modify the entry if it is already present.
  .and_modify(new_is_cross_function)
  // Or insert a new value.
  .or_insert(new_is_cross_function);

[Yoric]

Nit: Any chance you could replace function_scope with an enum? This would make things a bit more readable for callers.

Also, if it's a boolean, I'd rather call it is_function_scope.

[Yoric]

Nit: A comment explaining what you're doing here would be nice.