Remove MaybeDone from impl Join for Vec

[yoshuawuyts]

This is the first step towards #22. Conceptually it moves us from the existing structure, which has all things together in a single enum:

pub struct Join<Fut: Future> {
    state: Pin<Vec<MaybeDone<Fut>>>>,
}

Towards a separated input/output/metadata structure, which has far better locality:

pub struct Join<Fut: Future> {
    #[pin]
    futures: Vec<Fut>,
    items: Vec<MaybeUninit<<Fut as Future>::Output>>,
    metadata: Vec<Metadata>,
}

This requires some unsafe at the end to transmute the MaybeUninit<T> to T. Which also requires us to mess around with custom drop implementations. But as a result, it should have better locality since we only ever have to write the data once.

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Because this is the first iteration of a series of PRs, it has a lot more things going on. We introduce a new type: Metadata, which tracks the state. Unlike MaybeDone which wraps a T, Metadata is expected to be co-located with the state. Each piece of Metadata carries an enum State, which tracks what state the futures and output are in.

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r? @eholk, this ties into some of the earlier work we've been doing. Tagging you so you can take a look if you're interested!