Make try_log public so that custom loggers can use it

[Boscop]

My use case that requires this change is:
From my DLL plugin I want to log to a file but also to the host, by sending it strings (one per log record).
I need to log each line separately to the host, but Write doesn't let me know when each record starts/ends.
So I need to log to a String (actually Cursor<Vec<u8>>, then str::from_utf8, then send the String to the host) and I need to be able to call try_log for that.

[Drakulix]

In principle I don't mind to do this, but right now try_log has no documentation.

If you could try to add those (and make sure to explain potential use-cases and reasons this function is public) I would gladly merge this!

[Boscop]

I'm not sure what kind of doc to add.
In my use case, my DLL logger has to call try_log and then logs to the host via host_log.
(I need to log each line separately to the host, but Write wouldn't let me know when each record starts/ends.)

Like this:

pub struct HostLogger {
	level: LevelFilter,
	config: Config,
}

impl HostLogger {
	pub fn new(log_level: LevelFilter, config: Config) -> Box<HostLogger> {
		Box::new(HostLogger { level: log_level, config })
	}
}

impl Log for HostLogger {
	fn enabled(&self, metadata: &Metadata<'_>) -> bool {
		metadata.level() <= self.level
	}

	fn log(&self, record: &Record<'_>) {
		if self.enabled(record.metadata()) {
			let mut s = Vec::new();
			let _ = try_log(&self.config, record, &mut Cursor::new(&mut s));
			let s = std::str::from_utf8(&s).expect("utf8").trim();
			host_log(s);
		}
	}

	fn flush(&self) {}
}

impl SharedLogger for HostLogger {
	fn level(&self) -> LevelFilter {
		self.level
	}

	fn config(&self) -> Option<&Config> {
		Some(&self.config)
	}

	fn as_log(self: Box<Self>) -> Box<dyn Log> {
		Box::new(*self)
	}
}