Arrow functions with concise body not instrumented correctly

For instance, a function that reads `x => 5` will be transformed to ```x => { $_$twiz(/*...*/); 5 } ```, which returns undefined and not 5.

Failing integration test case:

```typescript
it('should correctly transform arrow functions that return arrow functions', () => {
  const input = `(x=>y=>x+y)(10)(5)`;
  expect(typeWiz(input)).toBe(`((x: number)=>(y: number)=>x+y)(10)(5)`);
});
```
