reftable: reading/writing blocks

The reftable format is structured as a sequence of block. Within a block,
records are prefix compressed, with an index of offsets for fully expand keys to
enable binary search within blocks.

This commit provides the logic to read and write these blocks.

Helped-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: hanwen

Makefile

@@ -2458,10 +2458,12 @@ xdiff-objs: $(XDIFF_OBJS)
 
 REFTABLE_OBJS += reftable/basics.o
 REFTABLE_OBJS += reftable/error.o
+REFTABLE_OBJS += reftable/block.o
 REFTABLE_OBJS += reftable/blocksource.o
 REFTABLE_OBJS += reftable/publicbasics.o
 REFTABLE_OBJS += reftable/record.o
 
+REFTABLE_TEST_OBJS += reftable/block_test.o
 REFTABLE_TEST_OBJS += reftable/record_test.o
 REFTABLE_TEST_OBJS += reftable/test_framework.o
 REFTABLE_TEST_OBJS += reftable/basics_test.o