Extension trait for Sink

[fan-tom]

Currently, you cannot convert Sink::Error with .context. I suggest to add extension trait, similar to TryStreamExt, for Sinks, to make error conversion more convinient.

[shepmaster]

That seems reasonable. I was under the impression that Sinks were semi-deprecated, which is why I didn't add support for them originally.

General shape of how this would work:

1. Add sink.rs to src/futures01/ and src/futures/, importing them from the relevant modules.
2. Tests should be added to /compatibility-tests/futures-0.1 and /compatibility-tests/futures

[fan-tom]

Sink trait in futures 0.3 is placed under futures-sink crate, while futures reexprots it. Snafu already depends on futures (internal-dev-dep), should I hide extension trait behind feature flag, or make futures dependency mandatory?