make a html link in the doc comments

This seem to be what is now expected on stable; as the compiler warnings are saying; and also consistent with what `read_user_from_stream` example is using.

Update example to use Result<User, Box<dyn Error>>

`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

use from_utf8_unchecked in the fmt::Display implementation of Value