Integrate data type enums & Add SIMD support for unt8_t and int8_t Indices

[BlaiseMuhirwa]

The main goal of this relatively large PR is to allow us to build FlatNav indices of type other than FP32. Currently, with this PR, we support building Uint8 and Int8 indices as well.

Some important changes here include:

• The distance classes are now templated on datatype Ex.

SquaredL2Distance<DataType::uint8>(dim)

• The python classes are no longer under a common interface. This is not ideal, but I couldn't find an easy way to get a common interface without sacrificing performance. We now have indices of type, such as IndexL2Float, IndexIPInt8, etc. I linked an issue for fixing this in a subsequent PR: unified interface for all index types  #66
• Removed the function pointer design for computing distances. With reliance on template meta-programming for compile-time dispatching (and some runtime dispatching for selecting the right SIMD function implementation), we eliminate the need for a function pointer. This is mostly for code readability and extensibility.