Possible double free in YAML dump

[herwinw]

A bit of background: I was trying to find out why some specs in the nightly runner (e.g. core/string/dump_spec.rb) appear as if they are completely skipped (0 passed, 0 failed, 0 errored). It turns out the YAML formatter can create invalid YAML due to a lack of character escaping in String#inspect. Even with the improvements of #1838 it still has a number of issues.

Given that we manually print the YAML output, the easiest thing to do was using the YAML library, but the first implementation resulted in double free errors (at least on my system).

This branch currently is a research branch to dive deeper into that issue.