Add support for write! macro to Serializer

[nyurik]

The write!($dst, $args+) macro wraps a call to $dst.write_fmt(format_args!($($arg)*)) - making it ergonomic to use formatted output on anything "writable" -- write!(dst, "foo={foo}"). It seems serde does not support write! because it does not have write_fmt, even though it has almost identical collect_str. Even its documentation implies similar usage pattern.

Would it be possible to add another write_fmt fn to the Serializer to make this usecase easier? In theory, if collect_str was called write_fmt, it would have worked out of the box, so a default Trait impl to simply forward the call should be possible?