LSH Index + Index I/O module

New

• Index type LshIndex for locality sensitive hashing indexing.
• index::io module for reading and writing indices from/to disk.

Breaking changes

• New error variant BadFilePath for incompatible file path strings in io module.
• Relies on an updated C API of faiss_c, so you may need to rebuild Faiss with the latest version.

Minor tweaks

• implement Debug for StandardGpuResources