Demo R package. Shows how to call R from C

• .Call to get to C from R and eval to call R from C

It used to show how to use

• .C to get to C from R and call_R to call R from C

but the latter, which had been deprecated since R-2.15.0, was finally removed from the R API in R-4.2.0. It can no longer be used and has been removed from this package.

Also shows how to call R from Fortran (via C function pretending to be Fortran subroutine).

This package when checked with

R CMD build qux
R CMD check qux_*.tar.gz --as-cran

generates complaints about registration of native routines. If we actually wanted to make this a CRAN package (which of course we don't because it is just a demo), we would need to do that. See R package fooRegister in Github repo foo (https://github.com/cjgeyer/foo) for how to do that.