Builtins now have visibility controlled through their annotation and
other nodes explicitly call RContext.setVisibility

FastR: an implementation of R in Java atop Truffle and Graal. Contributed by Tomas Kalibera, Petr Maj, and Jan Vitek (Purdue University); Christian Humer and Andreas Woess (Johannes Kepler University Linz); Michael Haupt, Mick Jordan, Adam Welc, Christian Wirth, and Thomas Wuerthinger (Oracle Labs).