Supporting n-ary `join`

[rocallahan]

I want to run N closures in parallel, where each closure returns a value of a different type. join works great for two tasks. I can use join for N tasks by building a tree of binary joins, but that's ugly.

Would it make sense to have a join_all! macro that does an N-ary join, building a linear chain of joins under the hood? Or is there a better way to implement it?

[cuviper]

There's an old PR for that, #406. That included a funky inner-let form that we were a little worried about, but a simple variadic join! that returns a flat tuple seemed like a safe bet.

There are some macros added since in src/iter/multizip.rs that might help with tuple flattening. We probably don't want to export internal utility-level stuff though.