Encode compression type in Nippy header, major refactor/housekeeping

Housekeeping includes:
  * Importing useful encryption+compression stuff into primary ns
    for lib consumers.
  * Promoting a number of things from Alpha status.
  * Exceptions are now all `ex-info`s.
  * Simplification of `thaw` API: Nippy v1 support is now automatic
    & configuration-free (performance impact in most cases is negligible).
Update benchmarks
v2.7.0-SNAPSHOT