reftable/constants: make block types part of the public interface

reftable/constants: make block types part of the public interface

Now that reftable blocks can be read individually via the public
interface it becomes necessary for callers to be able to distinguish the
different types of blocks. Expose the relevant constants.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>