format: implement implicit format instance upgrades facility
    
    Quoting part of the new docstring extension:
    > implicit forward compatibility takes the least-resistance
    > approach -- no hassles are needed when an instance of an older
    > format version validates per requirements imposed on the newer
    > format, otherwise this new format is supposed to specify
    > filter(s) sufficient to promote the older instance so it
    > eventually validates
    
    Originally, it was assumed the peripety will take place in
    as_instance method, but for truly universal implicit upgrade
    we need to occupy the very format instantiation.
    
    Signed-off-by: Jan Pokorný <jpokorny@redhat.com>