Expose raw parse tree for zero-copy Rust bindings

[paupino]

There may be a way to reduce dependencies further as well as reduce layers of complexity, by extrapolating the pg source directly. This could perhaps leverage the ruby script that libpg_query already uses.

[paupino]

I think a more interesting direction: expose libpg_query's pg_query_raw_parse (currently internal - see src/pg_query_parse.c) and add an entry point that returns the raw List* tree together with its MemoryContext, without tearing the context down. On the Rust side, bindgen the internal node headers (already vendored under src/postgres/include/nodes/) and build a zero-copy view whose lifetime is tied to the arena, dropping the context on Drop.