grammars: 1.5x faster inference w/ complex grammars (vector reserves / reuses) (#6609)

* grammars: reserve rejects & next candidates

* grammars: reuse new_stacks

* grammars: fix missing sig change in llama.h

* grammars: fix test (api changed)

* grammars: update gbnf-validator.cpp

* grammars: simpler syntax (no swap)