Convert to proper YAML library

[osa1]

So, standard compliance is good, but in this case we care more about stack compatibility than standard compliance, and stack currently uses yaml.

That said, as a user I won't complain unless stack accepts some stack.yaml files that stack2cabal rejects.

[edsko]

@hvr are you aware of any stack.yaml in the wild that cannot be parsed by the standards compliant parser?

[hvr]

I'm not aware of any... and I'm not sure how I can get my hands on a large enough quantity of stack.yaml files; fwiw, https://github.com/erikd/jenga is also using HsYAML for parsing stack.yaml files... and I haven't heard any complaints yet.

[edsko]

Ok.