stream: add StreamExt::timeout()

[alce]

Adds a per-item timeout to Stream

[hawkw]

@alce a couple of notes:

1. it looks like the feature flagging of the timeout method to depend on the "time" feature isn't complete?
2. looks like the basic_usage test has failed on MacOS but passed on Linux...I wonder if, rather than there being an OS-related failure, the test is flaky/non-deterministic?

[alce]

Thank you @hawkw. I fixed the flaky test and feature-gated as suggested.

[alce]

There are 2 things I didn't think of when implementing this that may make sense:

• reuse the Timeout struct that is already in the time module and implement Stream on that instead of introducing a new struct

• keep an additional global timer or counter to stop polling if the inner stream gets stuck for too long on an individual item. If this makes sense it would probably be necessary to introduce a different error, maybe ItemElapsed?

[carllerche]

Looks great 👍 I had a behavior question I wrote up inline.

[carllerche]

Looks good to me 👍 thanks for sticking w/ this.

[hawkw]

lgtm, modulo one docs nit!