Introduce [pure] as coeffects bottom type

The current coeffects system allows you to describe a pure function by using the empty set `[]` annotation:

```hack
function someFunc(int $x)[] {
return $x + 1;
}
```

While this makes sense given the system described, it's easy to miss when scanning code:

```
function someFunc(int $x)[] { // no effects allowed
vs
function someFunc(int $x) { // all effects allowed
```

What if you instead required an explicit pure keyword:

```hack
function someFunc(int $x)[pure] {
return $x + 1;
}
```

It would be the bottom type of the system, so `[pure, io]` would be treated as `[io]` (and would presumably also be flagged by the typechecker).