Add support for toml?

I intend to use chiptool to generate a generic usb OHCI register block, but I noticed that multi-line descriptions are turned into a single line.

I tracked the problem to serve_yaml, which doesn't preserve multiple lines, and couldn't find a alternate yaml parser that supports serde.
After implementing toml support in the `gen-block` command I also had to fix `utils::respace`, which was eating newlines.
Now I have a way to generate multi-line comments. 👍

Are you guys interested in supporting toml in the various commands?
I can make a PR, it would be trivial to add other serde formats (like json) after that.