0.3

The 0.3 release brings 3 "usability improvements" to STM and Util
and a Lin search improvement that should reduce memory allocation.

• #400: Catch and delay exceptions in STM's next_state for a nicer UX
• #387: Reduce needless allocations in Lin's sequential consistency
  search, as part of an Out_channel test cleanup
• #379: Extend the set of Util.Pp pretty-printers and teach them to
  add break hints similar to ppx_deriving.show; teach to_show to
  generate truncated strings when $MCTUTILS_TRUNCATE environment
  variable is set
• #368: Switch STM_domain.agree_prop_par_asym from using
  Semaphore.Binary to using an int Atomic.t which improves
  the error rate across platforms and backends