SuperKMeans and Full Refactor

[lkuffo]

Changes

• Replacing FAISS with SuperKMeans for index building. Expect 10-30x faster index building. In doing so, we eliminated the dependency on FAISS and inefficient Python scripts.
• C++ functions for PDXification in 32-bit and 8-bits.
• Several bugs and memory leak fixes.
• Support for U8 in AVX2.
• Removing unnecessary code and reducing benchmarks.
• Adding unit tests and formatting.
• Adding CI suite
• PDX is now a much nicer library with nicer API