use from_utf8_unchecked in the fmt::Display implementation of Value #751
Since the serializer only emits valid utf8 by default we can remove the checks that the data being formatted is valid.
this speeds up formatting large
`WriterFormatter` has been moved into it's own little module to enforce using it's `new` method to convey the unsafe requirements. the safety requirements that the serializer only emits valid utf8 is already assumed by the existing `to_string` methods. this speeds up formatting large `Value`s by about 33% in my testing