Fix FmtToIoWriter::write_str to call write_all
Fix FmtToIoWriter::write_str to call write_all

Previously, the implementation could silently fail to write some
bytes. Now, the implementation should either write all the bytes or
return an error.

The docs for `std::fmt::Write::write_str` say, "This method can only
succeed if the entire string slice was successfully written, and this
method will not return until all data has been written or an error
occurs." So, `FmtToIoWriter::write_str` must call
`std::io::Write::write_all` instead of `std::io::Write::write` on the
inner writer in order to ensure that all the bytes are written.

dtolnay/serde-yaml