This PR is intended to supersede just-the-docs#462.

The only user-level difference from just-the-docs#462 is that disambiguation of parent pages has to use either `grand_parent` or `ancestor` titles: the somewhat unnatural `section_id` and `in_section` fields are not supported.

The implementation has been significantly simplified by the changes introduced in v0.7.0 of the theme.

This initial draft has not yet been rigorously tested nor profiled.

Unclear to me why adding this gem causes CI failures.

A page should not have a parent or ancestor with the same title. If it does, the location of the repeated link is marked by ∞, to facilitate debugging the navigation (and an unbounded loop leading to a build exception is avoided).

It might be preferable to emit a warning in the Jekyll build log, but that doesn't appear to be supported by Liquid/Jekyll tags.

Reuses `nav_breadcrumbs`.

In Jekyll 3, Liquid’s binary `and` and `or` operations cannot be used in the `where_exp` filter...

Avoid diffs from sites built with v0.8.0

For backward compatibility with 3-level sites

For backward compatibility with 3-level sites

This is for backwards compatibility with sites that use `has_children: false` to suppress navigation links to child pages.

Takes account of ancestors.

To test, add to 404.html front matter:

```yaml
parent: T
grand_parent: S
ancestor: Y
```

Then check that 404.html has the correct breadcrumbs.

When activated by `nav_error_report: true` in `_config.yml`, displays warnings about pages with the same title as their parent page or an ancestral page.

The breadcrumbs for excluded pages were incorrect in previous commits, due to interference between assignments in recursive inclusions of ancestry.html.

It appears that:
- `nav_levels[1] == nil` holds only when not `site_nav contains nav_list_link`;
- `nav_levels[2] == nil and nav_levels[3]` never holds.

* Generate rules for any number of levels
* Improve layout of generated rules
* Improve layout of Liquid tags

Revert to the original code for nav/ancestry.

The previous draft increased the build time 5x with Jekyll 3 for a site with about 550 pages, of which 150 are excluded from the main navigation.

The extra cached site-nav is used for determining breadcrumbs and children navigation, which may involve pages that are excluded from the main navigation.

The inefficient code previously used for excluded pages is now redundant, and to be removed in a subsequent commit.

Replace code for determining children by inclusion of components/nav/children.html

Profiling locally, this doesn't appear to lose efficiency.