registrar: introduce new class for registration to replace funcregistrar _funcregistrarbase differs from funcregistrar in points below: - every code paths should use same class derived from _funcregistrarbase to register functions in a same category funcregistrar expects (3rd party) extensions to use (a class derived from) delayregistrar. - actual extra setup should be executed in another function For example, marking revset predicate as "safe" is executed in a class derived from _funcregistrarbase, but putting name of "safe" predicate into safesymbols is executed in another function for it. funcregistrar expects derived classes to do so. New class is named as module private one, because code paths, which register functions, should use not it directly but one derived from it.