* lang/cpp/src/context.cpp
(Context::revUid, Context::startRevUid),
(Context::addUid, Context::startAddUid): New.
* lang/cpp/src/context.h: Declare new functions.
* lang/cpp/src/key.cpp (Key::UserID::revoke)
(Key::addUid): Idomatic helpers.
lang/cpp/src/key.h: Declare new functions.
* NEWS: Update accordingly.