Use conformant C API of R

[lionel-]

Result: NOTE
  File ‘magrittr/libs/magrittr.so’:
    Found non-API calls to R: ‘Rf_allocSExp’, ‘Rf_findVarInFrame3’,
      ‘SET_ENCLOS’, ‘SET_PRCODE’, ‘SET_PRENV’, ‘SET_PRVALUE’

• Rf_allocSExp() (1/2) and SET_ENCLOS():
  Use the new R_newEnv() from R 4.1.0 (r-devel/r-svn@434646b).

• Rf_findVarInFrame3():
  Use R_existsVarInFrame() from R 4.2.0 (r-devel/r-svn@5413a63) and R_getVar() from R 4.5.0 (r-devel/r-svn@29a2fda). Or just use R_getVarEx() from the same version to check for existence.

• Rf_allocSExp() (2/2), SET_PRCODE(), SET_PRENV(), SET_PRVALUE():
  Use new promise constructor from https://gist.github.com/lionel-/1ebcbd5ec69c0775d514c329522408a3