Wip/jsuereth no blankies

In addition to merging 0.13 inot the codebase, this PR adds:

* A new internals namespace for parser details to not leak in public API
* A return (and deprecation) of public methods so we don't break binary compatibility
* Moving things into private[sbt] to avoid exposing APis we'll need to alter in the future when fixing bugs.
* A set of acceptance tests for `session save` functionality (one currently fails)

Review by @ajozwik cc> @eed3si9n