Re-export only things that are used by other modules.
In the future, I'm imagining that we run bindgen twice, for Postgres
v14 and v15. The two sets of bindings would go into separate
'bindings_v14' and 'bindings_v15' modules.
Rearrange postgres_ffi modules.
Move function, to avoid Postgres version dependency in timelines.rs
Move function to generate a logical-message WAL record to postgres_ffi.
* Deduce `last_segment` automatically
* Get rid of local `wal_dir`/`wal_seg_size` variables
* Prepare to test parsing of WAL from multiple specific points, not just the start;
extract `check_end_of_wal` function to check both partial and non-partial WAL segments.
