Checklist for v1.2.0

[joshprk]

Since this is being used for pkgs.formats.kdl in nixpkgs, we should fix a few things to polish up the tool. I'm mostly writing this for my own reference later, but feel free to send PRs if you'd like to tackle some of these.

• Expose json to kdl logic to a public interface (lib.rs), and call it from the thin cli (main.rs).
• Use Write trait instead of multiple different fd abstractions to simplify logic for stdout vs. file
• Remove unnecessary memory allocations in logic

I know the json dsl might have a few limitations, I'd appreciate if people could post the most pressing limitations that bother them which I could possibly implement a solution for here (if there are any).

EDIT: Due to lack of time on my end and helpful PRs which I feel should be upstreamed, I'm moving these plans to 1.2.0