Remove implicit conversion operators on sexp

[DavisVaughan]

cpp11 currently allows some somewhat scary implicit conversions between sexp and the 3 types bool, size_t, and double. These are unchecked conversions, and we'd like to remove them. However, doing so breaks readxl, vroom, and supercells, so we sent them PRs for this:

• Avoid scary implicit conversion from sexp to bool tidyverse/readxl#757
• Avoid implicit conversion from sexp to bool tidyverse/vroom#543
• Avoid implicit conversion from sexp to double Nowosad/supercells#39

We can merge this once those packages have merged and updated on CRAN