ggml : quantization refactoring (#3833)
* ggml : factor all quantization code in ggml-quants
ggml-ci
* ggml-quants : fix Zig and Swift builds + quantize tool
ggml-ci
* quantize : --pure option for disabling k-quant mixtures
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Co-authored-by: cebtenzzre <cebtenzzre@gmail.com>
