core: Continue PieCrust3 refactor, simplify pages. The asset pipeline is still the only function pipeline at this point. * No more `QualifiedPage`, and several other pieces of code deleted. * Data providers are simpler and more focused. For instance, the page iterator doesn't try to support other types of items. * Route parameters are proper known source metadata to remove the confusion between the two. * Make the baker and pipeline more correctly manage records and record histories. * Add support for record collapsing and deleting stale outputs in the asset pipeline.

---
config:
    site:
        root: /~john/public/
in:
    pages/about.md: 'URL: {{page.url}}, LINK: {{pcurl("missing")}}'
    pages/_index.md: 'URL: {{page.url}}'
out:
    about.html: 'URL: /%7Ejohn/public/about.html, LINK: /%7Ejohn/public/missing.html'
    index.html: 'URL: /%7Ejohn/public/'