reftable/block: expose a generic iterator over reftable records

reftable/block: expose a generic iterator over reftable records

Expose a generic iterator over reftable records and expose it via the
public interface. Together with an upcoming iterator for reftable blocks
contained in a table this will allow users to trivially iterate through
blocks and their respective records individually.

This functionality will be used to implement consistency checks for the
reftable backend, which requires more fine-grained control over how we
read data.

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