llama : std::move llm_bigram_bpe from work_queue (#9062)

* llama : std::move llm_bigram_bpe from work_queue

This commit updates the retrieval of llm_bigram_bpe objects from
work_queue.top() by using std::move.

The motivation for this is to avoid the copying of the std::string
`text` member of the llm_bigram_bpe struct.

* squash! llama : std::move llm_bigram_bpe from work_queue

Introduced a MovablePriorityQueue class to allow moving elements
out of the priority queue for llm_bigram_bpe.

* squash! llama : std::move llm_bigram_bpe from work_queue

Rename MovablePriorityQueue to lama_priority_queue.

* squash! llama : std::move llm_bigram_bpe from work_queue

Rename lama_priority_queue -> llama_priority_queue.