Add support for IQn_K quants

[arnfaldur]

@ikawrakow has created new quantization schemes IQ2_K, IQ3_K, IQ4_K, IQ5_K (ikawrakow/ik_llama.cpp#8) and IQ6_K (ikawrakow/ik_llama.cpp#14) that together constitute a new pareto frontier, based on his perplexity tests.

Interestingly, he found that Q6_K caused considerable quality loss, contrary to popular belief, when quantizing newer models. Theorizing that they contain more information in their weights and are therefore more sensitive to quantizations. IQ6_K reduced that quality loss from 0.65% to 0.4%.

High quality qantizations, combined with relative ease of use, are the main differentiator llama.cpp has with it's competitors from my, and likely many others', perspective.

Supporting these new quantization schemes would be a great addition to the project and would help llama.cpp stay in it's class of it's own level of quality.

@ikawrakow already implemented all the quants in his fork ikawrakow/ik_llama.cpp#7 but it has diverged significantly.

There seem to have been some disagreements that lead to this fork being made. But I think everyone can understand the value in upstreaming this work, and finding a path that would lead to that happening.