IQ3_S: much faster CPU prompt processing

[ikawrakow]

As PRs #515, #516, #517.

Here a sweep-bench with this PR for LlaMA-3.1-8B on a Ryzen-7950X CPU

PP	TG	N_KV	T_PP s	S_PP t/s	T_TG s	S_TG t/s
512	128	0	1.733	295.36	8.239	15.54
512	128	512	1.805	283.62	8.398	15.24
512	128	1024	1.857	275.73	8.561	14.95
512	128	1536	1.905	268.74	8.430	15.18
512	128	2048	1.954	261.97	8.563	14.95

I haven't done this for a while, but I think for this one worth looking at mainline llama.cpp (build: 5635 (3069e3169))

PP	TG	N_KV	T_PP s	S_PP t/s	T_TG s	S_TG t/s
512	128	0	18.261	28.04	7.933	16.14
512	128	512	18.708	27.37	8.335	15.36
512	128	1024	19.048	26.88	8.547	14.98
512	128	1536	19.480	26.28	8.739	14.65
512	128	2048	19.670	26.03	8.912	14.36

10X faster PP here!