release rayon 0.8.1, rayon-core 1.2.0

[nikomatsakis]

• The following core APIs are being stabilized:
  • rayon::spawn() -- spawns a task into the Rayon threadpool; as it
    is contained in the global scope (rather than a user-created
    scope), the task cannot capture anything from the current stack
    frame.
  • ThreadPool::join(), ThreadPool::spawn(), ThreadPool::scope()
    -- convenience APIs for launching new work within a thread-pool.
• The various iterator adapters are now tagged with #[must_use]
• Parallel iterators now offer a for_each_with adapter, similar to
  map_with.
• We are adopting a new approach to handling the remaining unstable
  APIs (which primarily pertain to futures integration). As awlays,
  unstable APIs are intended for experimentation, but do not come with
  any promise of compatibility (in other words, we might change them
  in arbitrary ways in any release). Previously, we designated such
  APIs using a Cargo feature "unstable". Now, we are using a regular
  #[cfg] flag. This means that to see the unstable APIs, you must do
  RUSTFLAGS='--cfg rayon_unstable' cargo build. This is
  intentionally inconvenient; in particular, if you are a library,
  then your clients must also modify their environment, signaling
  their agreement to instability.