Remove calls to `Rf_findVarInFrame`

S7 currently has a check Note: ``` Version: 0.2.1 Check: compiled code Result: NOTE File ‘S7/libs/S7.so’: Found non-API call to R: ‘Rf_findVarInFrame’ Compiled code should not call non-API entry points in R. See ‘Writing portable packages’ in the ‘Writing R Extensions’ manual, and section ‘Moving into C API compliance’ for issues with the use of non-API entry points. ``` We also received an email with a deadline for when these should be addressed: ``` The NOTEs about calls to Rf_findVar and Rf_findVarInFrame will soon turn into WARNINGs, and we would like to remove these entry points in the 4.6.0 release, which will be released in late April. Can you please change to no longer use these entry points as quickly as possible? They have replacements documented in "Some API replacements for non-API entry points" of "Writing R Extensions": please change to use these replacements. Please correct before 2026-04-09 to safely retain your package on CRAN. Best wishes, The CRAN Team ```