Export underlying rtsk_* functions?

[gregorgorjanc]

As reported by https://github.com/HighlanderLab/RcppTskit/actions/runs/23729353545/job/69119640231 (and see below) we are showing/promoting access to some unexported rtsk_* functions via ::: because up to now we focused on exporting just the user-focused functions, but there is a growing number of underlying rtsk_* functions that are mostly meant to be used by developers, hence they are unexported for now (to avoid overwhelming users), yet we do want to make people/developers aware of the rtsk_* functions ...

So, we either tweak lines below to make jarl happy or export the rtsk_* functions. Both have pros and cons.

What are your thoughts on this @hannesbecher @LynxJinyangii ?

warning: 
   --> RcppTskit/vignettes/RcppTskit_intro.qmd:280:1
    |
280 | RcppTskit:::rtsk_treeseq_load
    | ----------------------------- Accessing a package's internal function with `:::` is likely to break in the future.
    |
    = help: Use public functions via `::` instead.

warning: 
   --> RcppTskit/vignettes/RcppTskit_intro.qmd:283:1
    |
283 | RcppTskit:::`_RcppTskit_rtsk_treeseq_load`
    | ------------------------------------------ Accessing a package's internal function with `:::` is likely to break in the future.
    |
    = help: Use public functions via `::` instead.

warning: 
   --> RcppTskit/vignettes/RcppTskit_intro.qmd:286:9
    |
286 | xptr <- RcppTskit:::rtsk_treeseq_load(ts_file)
    |         ----------------------------- Accessing a package's internal function with `:::` is likely to break in the future.
    |
    = help: Use public functions via `::` instead.

warning: 
   --> RcppTskit/vignettes/RcppTskit_intro.qmd:290:1
    |
290 | RcppTskit:::rtsk_treeseq_get_num_individuals(xptr)
    | -------------------------------------------- Accessing a package's internal function with `:::` is likely to break in the future.
    |
    = help: Use public functions via `::` instead.