log: make set_level(level Level) part of the log.Logger interface too (fix for discordv)

vlib/log/log.v

@@ -69,6 +69,7 @@ mut:
 	warn(s string)
 	info(s string)
 	debug(s string)
+	set_level(level Level)
 }
 
 // Log represents a logging object

vlib/log/log_test.v

@@ -84,6 +84,7 @@ fn test_logger_mutable_reference() {
 	println(@FN + ' start')
 	// get log as Logger and use it
 	mut logger := new_log_as_logger()
+	logger.set_level(.debug)
 	assert typeof(logger).name == '&log.Logger'
 	go logger_mutable_statements(mut logger)
 	delay() // wait to finish