Macro stack_let

macro_rules! stack_let {
    ($id: ident = $expr: expr) => { ... };
    (mut $id: ident = $expr: expr) => { ... };
    ($id: ident : $type:ty = $expr: expr) => { ... };
    (mut $id: ident : $type:ty = $expr: expr) => { ... };
}

stack_let!(id = expr) binds a PinStack<T> to id if expr is an expression of type U where T: FromUnpinned<U>.

If expr is of type Unpinned<U, T> for some U, then no type annotation is necessary. If expr is of type U where T: FromUnpinned<U>, use stack_let!(id : T = expr).

To bind id mutably, use stack_let!(mut id = expr).