Re-architect around a YARD parser

Start building a YARD parser. Parsing and comment extraction has been moved out
of the yardoc face and into the new `Puppetx::Yardoc::YARD::PuppetParser`
class. The old `Commentor` class from the Util module has been renamed to
`Puppetx::Yardoc::Pops::YARDTransformer`.

`puppet yardoc` is still capable of extracting docstrings for node, class and
type definitions from a single manifest file.
Add YARDStatement class

The `YARDStatement` class is an adaptor that composes a `Pops::Model` instance
along with applicable `Pops::Adapter` and presents a familiar interface for
`YARD::Handler` objects to work with.