Proposal: rename Bytes::to_string to Bytes::to_string_unchecked

[Li Junchen]

• Add a new method Bytes::to_string_unchecked that behaves the same as the current Bytes::to_string.
• Change the behavior of Bytes::to_string to output bytes literal or bytes array.

Related: #257

[Hongbo Zhang]

We should add bytes literal syntax,e.g, b"xx.." and use make Byte::to_string confirm to the format