reftable/block: create public interface for reading blocks

While users of the reftable library wouldn't generally require access to
individual blocks in a reftable table, there are valid usecases where
one may require low-level access to them. One such upcoming usecase in
the Git codebase is to implement consistency checks for the reftable
library where we want to verify each block individually.

Create a public interface for reading blocks. The interface isn't yet
complete and lacks e.g. a way to read individual records from a block.
Such missing functionality will be backfilled in subsequent commits.

Note that this change also requires us to expose `reftable_buf`, which
is used by the `reftable_block_first_key()` function.

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

reftable/basics.h

@@ -18,13 +18,6 @@
 
 #define REFTABLE_UNUSED __attribute__((__unused__))
 
-struct reftable_buf {
-	size_t alloc;
-	size_t len;
-	char *buf;
-};
-#define REFTABLE_BUF_INIT { 0 }
-
 /*
  * Initialize the buffer such that it is ready for use. This is equivalent to
  * using REFTABLE_BUF_INIT for stack-allocated variables.