up version

(cargo-release) async-scoped version 0.7.1

Update version 0.7.0

Release: 0.6.1

Improve multiple executor support

+ allow spawn handle to return a different type than the
  spawned future's return value (used by tokio::spawn)
+ update to tokio 1.0

Published v0.5.1 to crates

Minor optimizations and bug-fixes

+ use `slab::Slab` for wakers (inspired by async-std)
+ refactor `Cancellation` concerns
+ lock-free read of cancellation status
+ better variable names!
+ hide cancellation until API matures

Move to FuturesUnordered

Better performance when spawning CPU-intensive non-uniform
futures with back-pressure.

Add into_stream to Scope