Refactor `sim_gs_n()` to improve interface and performance

[nanxstats]

It would be great to leverage the functional programming patterns available in R, such as function factories, to avoid the explicit quoting and evaluation in the current implementation.

The goal is to enable users to use the established functions and extend them freely following a specification.

This also involves data.table rewrite for the current dplyr implementation.

This is a follow up to #195.