buf: Add BufStreamExt trait and add a core feature (#897)

This change adds an extension trait to `BufStream` and puts the core
trait behind a feature flag for optional use.

This mainly adds the additional functions in an extension trait to
allow the user to select if they want just the core trait or the fully
featured version. Now the user can add the core feature to _not_
include the extension trait. By deafult, this feature is disabled.

.travis.yml

@@ -102,6 +102,7 @@ script: |
       # Limit the execution time of loom tests.
       export LOOM_MAX_DURATION=10
       cargo test --all --no-fail-fast
+      cargo test -p tokio-buf --no-default-features
       cargo doc --all
   fi