Optimize Content::has, Introspector::query_label, and MetaElem

[Dherse]

See the title.

It allows has to not allocate which while a small optimization on large documents and templates should really add up.

It also optimizes Introspector::query_label to be reference based, this does several things:

• Removes cloning for RefElem that are actually CiteElem
• Removes one clone in RefElem when it's an actual reference
• Removes all cloning in FootnoteElem::declaration_location

Also makes MetaElem be Unlabellable to make it smaller. I'd go as far as making it a custom impl without location, span, etc. too

[Dherse]

The difference these small changes makes is actually surprising, when editing references (i.e bibliography & headings) it actually provides a surprising 11% decrease in incremental compile time, so a bigger win than one might expect, likely due to the reduced amount of cloning. It also provides a small bump in cold compile of about 8% on masterproef which also surprises me, but we take the gains we get.

[laurmaedje]

Thank you!