Add Embeddings context manager

[davidmezzetti]

To make resource management easier, add __enter__ and __exit__ methods to the Embeddings instance. The __exit__ method should call close.

This change will enable the following patterns.

with Embeddings() as embeddings:
  embeddings.index(...)
  embeddings.save(path)

with Embeddings().load(path) as embeddings:
  embeddings.search(query)