Stream save llama context data to file instead of allocating entire buffer upfront (#2488)

* added stream saving context data to file to avoid allocating unnecessary amounts of memory

* generalised copying state data to file or buffer

* added comments explaining how copy_state_data works

* fixed trailing whitespaces

* fixed save load state example

* updated save load state to use public function in llama.cpp

* - restored breakage of the llama_copy_state_data API
- moved new logic for copying llama state data to internal function

* fixed function declaration order

* restored save load state example

* fixed whitepace

* removed unused llama-util.h include

* Apply suggestions from code review

Co-authored-by: slaren <slarengh@gmail.com>

* Apply code review suggestions

Co-authored-by: slaren <slarengh@gmail.com>

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Co-authored-by: slaren <slarengh@gmail.com>