reftable/block: rename `block` to `block_data`

reftable/block: rename `block` to `block_data`

The `reftable_block` structure associates a byte slice with a block
source. As such it only holds the data of a reftable block without
actually encoding any of the details for how to access that data.

Rename the structure to instead be called `reftable_block_data`. Besides
clarifying that this really only holds data, it also allows us to rename
the `reftable_block_reader` to `reftable_block` in the next commit, as
this is the structure that actually encapsulates access to the reftable
blocks.

Rename the `struct reftable_block_reader::block` member accordingly.

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