Use std::fmt::Write for string value writer

[Marcono1234]

Problem solved by the enhancement

The writer returned by JsonWriter::string_value_writer only supports writing valid UTF-8 data. However, currently it is a io::Write and therefore allows writing arbitrary bytes, which might not be valid UTF-8 data, and which therefore requires validation and buffering of incomplete / split UTF-8 data.

Enhancement description

Instead of using io::Write, use std::fmt::Write which only allows valid UTF-8 data. This would then avoid having to manually perform UTF-8 validation and handle incomplete UTF-8 data.
It would however lose the flush() method, but that was mainly implemented for the current implementation because io::Write includes it by default, not necessarily because it is really needed.

The question is though whether the string value writer should still in addition to std::fmt::Write also implement io::Write for the cases where users want to use a io::Write, for example when transferring data from a text file to the string value writer.

This was originally mentioned in https://users.rust-lang.org/t/error-handling-for-custom-read-read-write-write-implementation/101742/11