Switch from Rf_findVarInFrame to R_getVarEx

[eddelbuettel]

Closes #1421

As described in #1421, this is another upcoming R API tightening. This fairly narrow PR does this, keeping the old code til we can remove it a few years from now and switches to the new functions for R 4.5.0 or newer.

We should leave this open until the reverse dependency checks give us a view into what the impact may be.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[kevinushey]

Some nits around the value we pass for inherits.

[kevinushey]

Suggested change

	SEXP res = R_getVarEx(nameSym, env, TRUE, R_UnboundValue);
	SEXP res = R_getVarEx(nameSym, env, FALSE, R_UnboundValue);

I believe we want inherits = FALSE here since Rf_findVarInFrame() won't look through parent environments.

[eddelbuettel]

I actually had to trace around a test failure and flipped one of the other booleans. We will need to look at all of these. So appreciate the second set of eyes. Will change, and we should doube-triple check the others.

[eddelbuettel]

Done in 7636fec

[kevinushey]

Same as before.

[kevinushey]

Suggested change

	SEXP res = R_getVarEx(name, env, TRUE, R_UnboundValue);
	SEXP res = R_getVarEx(name, env, FALSE, R_UnboundValue);

[eddelbuettel]

Done in 7636fec

[kevinushey]

Suggested change

	Rcpp_cache = R_getVar(Rf_install(".rcpp_cache"), RCPP, TRUE);
	Rcpp_cache = R_getVar(Rf_install(".rcpp_cache"), RCPP, FALSE);

[eddelbuettel]

This one I was more torn about. Note the comment just above the code:

        /**
         * Get an object from the environment or one of its
         * parents
         *
         * @param name name of the object
         *
         */
        SEXP find( const std::string& name) const{

So does is walk through the other envs?

Also note that lines 139 to 162 (for find(std::string)) and 164 to 188 (for find(Symbol)) should be symmetric. Both TRUE, or both FALSE?

[eddelbuettel]

Rebased once more