Scaladoc hygiene for base package

As more stuff gets moved directly under `io.finch`, its API docs are getting a little unwieldy. It'd be nice to make prioritization traits `private[finch]` so they don't show up in the docs (their methods still will, though), and possibly even to organize the definitions using Scaladoc groups (see e.g. [how I'm using them in circe](https://github.com/travisbrown/circe/blob/master/core/shared/src/main/scala/io/circe/GenericCursor.scala)).

No files or tests are named. Start by locating the definitions directly under the io.finch package and reviewing their generated Scaladoc, then compare the grouping approach linked from circe. Done means prioritization traits are hidden with private[finch] where appropriate and the base-package documentation is organized into useful Scaladoc groups.

Written by the indexing model from the issue text.