reftable/block: rename `block_reader` to `reftable_block`

reftable/block: rename `block_reader` to `reftable_block`

The `block_reader` structure is used to access parsed data of a reftable
block. The structure is currently treated as an internal implementation
detail and not exposed via our public interfaces. The functionality
provided by the structure is useful to external users of the reftable
library though, for example when implementing consistency checks that
need to scan through the blocks manually.

Rename the structure to `reftable_block` now that the name has been made
available in the preceding commit. This name is in line with the naming
schema used for other data structures like `reftable_table` in that it
describes the underlying entity that it provides access to.

The new data structure isn't yet exposed via the public interface, which
is left for a subsequent commit.

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