0.6

CHANGES:

• add ppx_trace for easier instrumentation.
  • let%trace span = "foo" in … will enter a scope span named "foo"
  • let%trace () = "foo" in … will enter a scope named "foo" with a hidden name
• add trace-fuchsia backend, which produces traces in the binary format
  of fuchsia.
  These traces are reasonably efficient to produce (~60ns per span on my machines)
  and reasonably compact on disk, at least compared to the TEF backend.