Support HRTB

[emil-e]

Given the following declaration:

pin_project! {
    pub struct ReactiveControl<State, F, ViewControl>
    where
    F: for<'a> ReactiveFn<'a, State>,
    {
        f: F,

        #[pin]
        inner: OnceCell<ViewControl>,
        state_cell: RefCell<StateCell<State>>,
        _phantom: PhantomData<ViewControl>
    }
}

There is this error:

rustc: error: expected identifier, found keyword `for`
    --> retroact/src/reactive.rs:84:8
     |
  84 |     F: for<'a> ReactiveFn<'a, State>,
     |        ^^^ expected identifier, found keyword
     |
    ::: /Users/shadewind/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/pin-project-lite-0.2.16/src/lib.rs:1642:21
     |
1642 |                 $(: $where_clause_bound:path)?
     |                     ------------------------ while parsing argument for this `path` macro fragment

[taiki-e]

As with #62, I would accept a PR to improve parsing.