Add fold_keys and fold_kvs (key-value pairs) operations

[tatsuya6502]

Support common operations to iterate through keys or key-value pairs.

%% Start, inclusive
%% End, inclusive
-spec fold_keys(db(), fun((key(), AccIn::term()) -> AccOut::term()),
                Acc0::term(),
                StartKey::key() | undefined,
                StopKey::key() | undefined,
                NumItems::non_neg_integer()) ->
                       {ok, Acc1::term(), IsTruncated::boolean()}
                           | {error, io_error}.

%% Start, inclusive
%% End, inclusive
-spec fold_kvs(db(), fun(({key(), value()}, AccIn::term()) -> AccOut::term()),
               Acc0::term(),
               StartKey::key() | undefined,
               StopKey::key() | undefined,
               NumItems::non_neg_integer()) ->
                      {ok, Acc1::term(), IsTruncated::boolean()}
                          | {error, io_error}.