Implement `Send` and `Sync` for predicates that have `PhantomData` in them

[taminomara]

Predicates such as FnPredicate use PhantomData<T> to bypass the requirement that all generic parameters must be used. This makes the compiler think that predicate owns an instance of T, leading to two effects:

• if T is bound by lifetime 'a, then the predicate will also be bound by 'a,
• if T is not Send or Sync, the predicate will also not be Send or Sync.

There's nothing we can do about the first one. The second one, however, can be remedied by manually implementing Send and Sync.

For FnPredicate it will look like this:

unsafe impl<F, T> Send for FnPredicate<F, T>
    where
        F: Fn(&T) -> bool + Send,
        T: ?Sized,
{
}

unsafe impl<F, T> Sync for FnPredicate<F, T>
    where
        F: Fn(&T) -> bool + Sync,
        T: ?Sized,
{
}

This change will add unsafe to the crate. The benefit is that predicates will be easier to use in multi-threaded environments.

[epage]

Seems reasonable! Be willing to prep a PR for this?

[taminomara]

Sure, I'll try to send it later this week.