Implement full-fidelity parsing and formatting for the coroutine keyword

Implement full-fidelity parsing and formatting for the coroutine keyword

Summary:
Supports parsing the `coroutine` keyword as a modifier for function-like constructs in places where `async` is supported today:

1. Function declarations:
```
coroutine function test(): void {}
```

2. Anonymous functions:
```
function foo(): int {
  $x = coroutine function() {
    return 5;
  };
  return $x();
}
```

3. Lambdas:
```
function f(): (function(int): int) {
  return coroutine $v ==> 1;
}
```

4. Blocks:
```
function test(): void {
  return coroutine {
    return 1;
  };
}
```

In addition to parsing, this diff also implements formatting. It does //most// of the lowering as well. However, I have not yet updated `ast.ml` to include the new `coroutine` construct as a `fun_type`. This step has been left as a `TODO` in the code.

Reviewed By: ericlippert

Differential Revision: D4884746

fbshipit-source-id: 9489e9c11b4d9bcdca44b58a6c69cf2170c96fa0