8.9.0+0.6.0: Release 0.6.0 for Coq 8.9.0

CHANGES:

• [general] support Coq 8.9,
• [general] SerAPI now uses Dune as a build system,
• [opam] install sertop.el,
• [serlib] support to serialize kernel environments,
• [serapi] new query Env that tries to print the current kernel environment,
• [serlib] correct field names for CAst,
• [serlib] more robust support for opaque / non-serializable types (#61, #68).
  Thanks to @palmskog,
• [serlib] new option --exn_on_opaque to raise an exception on
  non-serializable types; closes #68, thanks to @palmskog,
• [serlib] serialization test-suite from
  https://github.com/proofengineering/serapi-tests, thanks to
  @palmskog,
• [sercomp] add --mode option to better control output,
• [sercomp] add compser for deserialization (inverse of sercomp)
  (@palmskog),
• [serapi] Allow custom document creation using the NewDoc call.
  Use the --no_init option to avoid automatic creation
  on init. (@ejgallego)
• [sercomp] Allow compilers to output .vo (@ejgallego , suggested by
  @palmskog)
• [sercomp] Serialize top-level vernaculars with their syntactic
  attributes (such as location) (@ejgallego)
• [serapi] Add Assumptions query, at the suggestion of @Armael
  (@ejgallego)
• [sercomp] Disable error resilience mode in compilers; semantics are
  a bit dubious see coq/coq#9204 also #94.
  (@ejgallego, report by @palmskog)
• [sercomp] Add check mode to compilers to check all proofs without
  outputting .vo. (@palmskog)
• [sercomp] Add "hacky" --quick option to skip checking of opaque
  proofs. (@ejgallego, request by @palmskog)
• [sercomp] Add --async_workers option to set maximum number
  of parallel async workers. (@palmskog)
• [sertop] Stop linking Coq plugins statically and load serlib
  plugins when Coq plugins are loaded instead (@ejgallego,
  review by @palmskog)