Cross-referencing package

[Omikhleia]

Rationale for this PR:

• Closes Referring to a section  #1007
• Moreover, the SILE Wiki mandated me to propose it...
  
  (so challenge taken, and I claim the bonus points too ;) )

What this packages proposes:

• Generic cross-references, with a LaTeX-inspired syntax (\label, \ref, \pageref...).
• An API for other packages or classes to support it (that is, register their own numbering items): here applied to the book class and footnotes package.

If it makes it into the distribution, I intend it as a replacement for my own "omirefs" package (it's the exact same code, the only change here being the documentation, using our new autodoc stuffs).

As for the TOC, it requires one or more re-run(s) of SILE. It doesn't come with tests (until we have a clean way to automate those re-runs), nevertheless, FWIW:

• Its own package documentation uses it.
• I used it in my own package and class documentations
• I used it in my book project... Including a gamebook-like chapter (of the "Choose Your Own Adventure" type).

[alerque]

I think we're 99% of the way towards making this package independent so it doesn't need to touch the class or other packages. Lets get it all the way there before merging. I've spent days recently trying to untangle other packages so they can be used in isolation and/or are idempotent to load. I think that's going to be a requirement for new core packages going forward and this one only needs minor tweaks to get there.

[Omikhleia]

Update: Rebased on current master at this date (7925d45) and upgraded to 0.14 API.

Caveat: In the manual, this is not working very well any longer, but for two root causes which IMHO are unrelated:

• the same package can be (re)initialized multiple times (i.e. re-instantiated in class:loadPackage), e.g. infonodes gets initialized 2 or 3 times in the manual, leading to Attempted to set the same function hook twice warnings very early when processing the manual).
• Autodoc is quite broken and/or is messing up (so here, tableofcontents re-register its commands, and labelrefs' tocentry override is disabled.

[Omikhleia]

The same package can be (re)initialized multiple times (i.e. re-instantiated in class:loadPackage), e.g. infonodes gets initialized 2 or 3 times in the manual, leading to Attempted to set the same function hook twice warnings very early when processing the manual).

Here, each time a package loads another one on which it is dependent, with self.class:loadPackage(), then a new instance is created.

[Omikhleia]

I am refactoring this as an "external" luarocks package.

[Omikhleia]

Here it goes https://github.com/Omikhleia/labelrefs.sile