The RegisterPrimOp class used to register builtins has been removed. Plugins
must now call PluginPrimOps::add from their nix_plugin_entry with the same
parameters previously passed to RegisterRrimOp to register any new builtins.
The GlobalConfig::Register helper class has also been removed. Adding config
options to the system is now done with GlobalConfig::registerGlobalConfig; a
plugin can add config values by calling this function from nix_plugin_entry.
