Raise diskann maximum dimension from 2K to 16K

[tjgreen42]

This PR fixes #100 and raises the dimension limit for pgvectorscale's diskann index from 2000 to 16000, which is the maximum supported by the underlying pgvector vector type.

The previous limit of 2000 was needed to ensure that all data structures could be serialized onto single 8K pages. When going beyond 2000 dimensions, so long as SBQ is used for storage, quantized vectors, neighbor lists, and other data structures will still fit on a single page; the only thing that grows too large is SbqMeans. (The raw vectors used for reranking remain in the source relation, where standard Postgres TOAST machinery is used to read/write them). If plain storage is used, the old limit of 2000 remains in place.

To deal with SbqMeans, we introduce a ChainTape data structure that is similar to Tape but supports reads/writes of large buffers across pages. The chained representation is considered a property of the PageType, and we introduce a new PageType for SbqMeans along with upgrade machinery from the old version. Similarly to the versioned MetaPage, there are no unit tests for this, but I did ad-hoc testing to confirm that the upgrade path works.

[cevian]

approved but left a few things to think about. Let me know if you decide to change anything and want another review.