speculative : add grammar support (#2991)
* speculative : add grammar support
* grammars : add json_arr.gbnf
* grammar : add comments to new grammar file
* grammar : remove one nested level
* common : warm-up with 2 tokens - seems to work better
* speculative : print draft token pieces
* speculative : reuse grammar parser + better logs and comments
* speculative : avoid grammar_mem
* make : fix speculative build
