Not enough unification of capture set between handler body and implementation

Consider the following program:

```
effect something(): Unit

def main(): Unit = {
val thunk = try {
do something()
box { return () }
} with something {
box { println("hello"); val _ = resume(()) }
}
thunk()
}
```

It fails to typecheck with "Not allowed {io}". I would like it to typecheck as written.

The following two variants do typecheck:

```
effect something(): Unit

def main(): Unit = {
val thunk = try {
do something()
box { return () } at io
} with something {
box { println("hello"); val _ = resume(()) }
}
thunk()
}
```

Where we explicitly annotate the `io` capture on the returned box.

Also, surprisingly:

```
effect something(): Unit

def main(): Unit = {
val thunk = try {
do something()
box { return () }
} with something {
box { println("hello") }
}
thunk()
}
```

Where we do not resume.