llama : refactor get / set state + remove redundant kv cache API

[ggerganov]

• Normalize the code style
• Move the definitions at the correct place in llama.cpp
• Retire llama_get_kv_cache(), llama_get_kv_cache_size() and llama_set_kv_cache()

Not sure how to test this - maybe we need to add an example, or extend main with store/load state functionality

[xaedes]

The change looks good to me, I love the readability improvements. The save_load script from #730 (comment) works as well.

I have converted this to an example with cmake already, will pull request it.

[ejones]

If it's helpful, I put up a take on save/load state in main in #1169 (mostly due to my impatience with 65B on the chat-13B prompt, hah).