Otherwise CURRENT_FEATURE_BUNDLE will end up including any hints added later that happen to use higher bits. This was causing autobox to turn off all features, causing failures for Dist::Zilla::PluginBundle::AVAR. I’m not adding tests for this, as such tests would need constant tweaking in future perl developement. What autobox is doing is naughty and unsupported anyway.
CURRENT_HINTS is not specific to features, but for now will be used by nothing else. It returns the compile-time or run-time hints, depend- ing on whether PL_curcop points to &PL_compiling. CURRENT_FEATURE_BUNDLE extracts the feature bundle number from the current hints.