reftable/reader: rename data structure to "table"

reftable/reader: rename data structure to "table"

The `struct reftable_reader` subsystem encapsulates a table that has
been read from the disk. As such, the current name of that structure is
somewhat hard to understand as it only talks about the fact that we read
something from disk, without really giving an indicator _what_ that is.

Furthermore, this naming schema doesn't really fit well into how the
other structures are named: `reftable_merged_table`, `reftable_stack`,
`reftable_block` and `reftable_record` are all named after what they
encapsulate.

Rename the subsystem to `reftable_table`, which directly gives a hint
that the data structure is about handling the individual tables part of
the stack.

While this change results in a lot of churn, it prepares for us exposing
the APIs to third-party callers now that the reftable library is a
standalone library that can be linked against by other projects.

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