Add Future::timeout

[yoshuawuyts]

Similar to future::timeout and Stream::timeout we should add a Future::timeout method.

We already have Future::delay, and timeout would be a great counterpart. This is similar to stdlib's max which is both an inherent method and a free function. Thanks!

[miker1423]

I like to take this if no one is working on it.

@miker1423 Go for it :)

[miker1423]

Sorry for the absence.
As I understood, I should add a new extension method to the Future trait?

[miker1423]

Here is the implementation miker1423@c1f7be5, I know the docs are incomplete, I just wrote to take away the warning. Is this what was expected?

[yoshuawuyts]

@miker1423 yeah that's in the right direction. I don't believe the pub modifiers are needed; pub(crate) should suffice. If you could submit the PR it'd make it easier to review. Thanks heaps for your work!

[miker1423]

This should be closed with the merged PR #600, thanks!