Avoid growing type parameters in Visitor

Avoid growing type parameters in Visitor

Summary:
`Visitor`, `Node` carries a list of type parameters, currently there are six, `Context`, `Error` and four aast annotations.

The long list of type parameters has lots of disadvantage,
1. Adding a new type parameters requires `O(n)` updates, `n` is number of all visitors,
2. Each visitor contains a lots boilerplate code,
3. Readability is compromised.

This diff reduces `n` type parameters to 1.

```
pub trait Params {
    type Context;
    type Error;
    type Ex;
    type Fb;
    type En;
    type Hi;
}
```

```
pub trait Node<P: Params> {
    fn accept(&self, ctx: &mut P::Context, v: &mut dyn Visitor<P = P>) -> Result<(), P::Error> {
        self.recurse(ctx, v)
    }
    fn recurse(&self, ctx: &mut P::Context, v: &mut dyn Visitor<P = P>) -> Result<(), P::Error> {
        Ok(())
    }
}
```

```
pub trait Visitor {
    type P: Params;

    ......

    fn visit_fun(
        &mut self,
        c: &mut <Self::P as Params>::Context,
        p: &Fun<<Self::P as Params>::Fb>,
    ) -> Result<(), <Self::P as Params>::Error> {
        p.recurse(c, self.object())
    }
}
```

Reviewed By: dabek

Differential Revision: D20333342

fbshipit-source-id: 7d8db5144a00a5113dfd5c8967dec753f646571d