Indexes on compressed data

[unroll]

Allow compressed indexes: an index that is built using compressed data, or built using the original that but stores compressed data for querying. Examples: Vanama+PQ, IVFPQ.

• Adapter to tie an index and compressor
• Internal changes to indexes and compressors to support compressed data storage
• Build index from compressed data
• Build index from uncompressed data, query on compressed data
• Assymetric distance computation (ADC) for PQ and similar

Stretch goals (not required to close issue):

• Optimize distance computation on compressed, uncompressed, and assymetric comparisons
• Support compressors with preprocessing steps such as OPQ and similar
• Improve BaseIndex / BaseCompressor APIs to support compressed indexes