grammar : pre-computed pieces + reserve mem + less string copies (#4330)
* reserve space for codepoints
* improvement for the appended 0
* used precomputed token text for grammar sample
* reserve canidates_decoded
* reserve canidates_grammar
* remove candidates_decoded
* Revert "remove candidates_decoded"
This reverts commit 3773328080e6a139ee83198329a13cf4ff61d707.
* changed decode_utf8 to take src by ref