log: set stderr as default log output, add .set_output_stream()

[gechandesu]

This PR:

• Adds set_output_stream() fn to log.Log struct which allows setting io.Writer as log output. It makes possible send logs to arbitrary targets such as sockets. Use cases: write logs to system journal e.g. syslog, send logs to remote server over UDP.
• Makes STDERR the default output stream (be STDOUT before). Sending logs to STOUT is not generally used in logging libraries e.g. Go, Python stdlibs etc.
• Edit test, since log.Log now have output_stream field with file address which can change at runtime and therefore cannot be tested in "inout" tests.

Example:

import log
import os
import net

fn main() {
	mut logger := log.Log{}
	logger.set_level(.info)
	log.set_logger(logger)

	logger.info('hello stderr')
	logger.set_output_stream(os.stdout())
	logger.info('hello stdout')

	mut udp_sock := net.dial_udp('127.0.0.1:8889')!
	logger.set_output_stream(udp_sock)
	logger.info('hello udp')
}

_{Huly®: V_0.6-21728}

[spytheman]

It failed locally the valgrind test, I am reopening it after the revert.

For some reason the test was not triggered by the last commit, which led me to believe that it was fixed. I should have investigated it closer.