Q4_1 quantization (#193)
* Add AVX2 version of ggml_vec_dot_q4_1
* Small optimisations to q4_1 dot product (@Const-me)
* Rearrange Q4_1 quantization to work for multipart models. (Fix #152)
* Fix ggml_vec_mad_q4_1 too
* Fix non-vectorised q4_1 vec mul
