Remove implicit conversion operators on `sexp`

[DavisVaughan]

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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:

• [ ] https://github.com/tidyverse/readxl/pull/757
• [ ] https://github.com/tidyverse/vroom/pull/543
• [ ] https://github.com/Nowosad/supercells/pull/39

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