* tokenizer: add SentencePiece-style BPE support
Add WithSentencePieceNormalizer option to BytePairEncoding for models
that use BPE with SentencePiece-style space markers (space to/from
U+2581).
NewBytePairEncoding is unchanged; the new NewBytePairEncodingWithOptions
constructor accepts BPEOption functions. Decoding handles the reverse
mapping of U+2581 back to spaces.
* review comments