ADDED SEXPTYPE in the altrep api

[CGMossa]

Last adjustment before we may adopt libR-sys enums in extendr.

• Non-exhaustive enum's are appearing as not FFI-safe

warning: `extern` fn uses type `libR_sys::Rboolean`, which is not FFI-safe
   --> extendr-api/src/wrapper/altrep.rs:845:23
    |
845 |                 narm: Rboolean,
    |                       ^^^^^^^^ not FFI-safe
    |
    = note: this enum is non-exhaustive

Of course, this may be an over-zealous linter, but I'm not capable of judging that myself. It seems pretty intensional.

• The Coerce method has a second argument which is an Rboolean, but it isn't specifically written like that in the R-api. Therefore, we adopt a custom version of this.
  See Consider changing this Altrep-interface #232 for more context
  Please note that this coerce thing is not used, or tested, or exposed in the extendr's AltRep API. I don't know what we should do about that.