0.4.0

Time for another (still experimental!) change to Diagnostic. It will
probably continue to change as miette gets experimented with, until 1.0.0
stabilizes it. But for now, expect semi-regular breaking changes of this kind.

Oh and I tracked down a rogue \n that was messing with the default reporter
and managed to get out of it with at least some of my sanity.

Breaking Changes

• protocol: Simplify protocol return values further (02dd1f84, breaks #)

Bug Fixes

• reporter:
  • fix reporter and tests... again (d201dde4)
  • fix extra newline after header (0d2e3312)