Don't box value in Fragile<T>

[Nilstrieb]

I'm not sure why the boxing was done here, but it isn't required for soundness and not doing it is more efficient so I've removed it. Maybe there's a good reason for doing it that I haven't considered.

[mitsuhiko]

I'm in favor of this change. I wanted to do this change before but I wasn't too comfortable with the behavior of SemiSticky. It's unclear if that type even makes too much sense, but if that change does happen it probably should Box there for the fragile case.

[mitsuhiko]

I'm actually not entirely sure about the change now. Today Fragile<T> boxes internally and in turn accomplishes that Drop is a noop for a lot of types. In the future Fragile<Box<T>> would invoke the dtor even in cases where it's not necessary because mem::needs_drop::<Box<T>>() would always return true. This severely limits the usefulness of Fragile as all drops in another thread for boxed values would abort the process.

[Nilstrieb]

People can just use Box<Fragile<T>> couldn't they?

[mitsuhiko]

I'm actually wondering now if this does not mean there is a memory leak in Fragile now for values without dtor. I need to validate this. You are probably right that one can just box the entire fragile instead.