Add stream::Chain trait

[yoshuawuyts]

This is a core iteration "order of execution" operation, enabling people to author:

before

// Manual sequential iteration.
for await n in 0..10 { dbg!(n); }
for await n in 11..20 { dbg!(n); }

after

// The same semantics using `chain`.
for await n in ((0..10), (11..20)).chain() {
    dbg!(n)
}

This is particularly useful to do things like "send one last message after EOF".

Design

pub trait Chain {
    /// What's the return type of our stream?
    type Item;

    /// What stream do we return?
    type Stream: Stream<Item = Self::Item>;

    /// Combine multiple streams into a single stream.
    fn chain(self) -> Self::Stream;
}

Tasks

• Chain trait Init zip and chain traits #73
• impl Chain for vec Init zip and chain traits #73
• impl Chain for array Init zip and chain traits #73
• impl Chain for tuple
• benchmarks