Adding HannoyTransformer

[amalia-k510]

This PR adds HannoyTransformer as proposed in #123, wrapping hannoy (LMDB-backed storage) into the sklearn-ann transformer interface. The approach follows the same pattern as AnnoyTransformer and others.

A few things to note:

• hannoy's Python bindings don't expose by_item yet (it exists in the Rust code in reader.rs but PyReader only has by_vec). A PR upstream was created to add it. Until that is approved, fit_transform stores the training data and re-queries it using by_vec instead of the faster by_item path. I plan to switch to by_item once it's available.

• There's a known issue where multiple Database instances in the same process silently share the first LMDB environment.

• I also opened an issue on hannoy requesting batch insert/query APIs to avoid the per-vector Python to Rust loop overhead.

Two questions: which metrics do we want to support? Right now it's only euclidean, but hannoy also has hamming, sqeuclidean, cosine, and manhattan. Also hannoy offers binary quantized variants of consine, euclidean, and manhattan. Would we want to also use those?

[flying-sheep]

There's a known issue where multiple Database instances in the same process silently share the first LMDB environment.

Do you mean a hannoy issue (if so, link plz) or in your code?

which metrics do we want to support?

All of them of course! Your code should not know which ones exist (except for the default): instead of accepting a string, just use the Metric enum as a type and pass that through to the upstream API.

[flying-sheep]

why is that?