Module(): analyse reference class once and reuse C++Class objects

[mdsumner]

Two changes in R/Module.R, inside Module()

#1514

speeds up load times, relevant especially to terra and gdalraster but affects other modules packages https://github.com/mdsumner/Rcpp/blob/code-docs-speedup-namespace-load/rcppmoduleloadtime.md

[Enchufa2]

Edit the ChangeLog too, please.