format source location messages to look like gcc messages

[soenkehahn]

Lots of tools (e.g. vim's quickfix) know how to parse gcc messages (and
allow you to jump to source locations).

See below for how the messages in this branch look like. I'm not overly passionate about the exact message format, but this seems to be the most logical to me and it works with vim's quickfix and geany.

Dev
  something
    foo FAILED [1]
    bar
      baz FAILED [2]
      boo FAILED [3]

Failures:

  dev/DevSpec.hs:15: (best-effort)
  1) Dev.something foo
       expected: False
        but got: True

  dev/DevSpec.hs:18: (best-effort)
  2) Dev.something.bar baz

  dev/DevSpec.hs:20: (best-effort)
  3) Dev.something.bar boo

Source locations marked with "best-effort" are calculated heuristically and may be incorrect.

Randomized with seed 1677688156

[sol]

@soenkehahn LGTM. Can you update the CHANGES.md and cut a release?

[sol]

@soenkehahn damn, we did not push this to master. Is the tag we created the same state as this PR?