Component manager will not always be able to rely on
fuchsia.logger.LogSink being present in its namespace, so this commit
adds new logging macros to src/log.rs to log to zircon's debuglog. If
debuglog creation fails, logs are sent to stderr.

CF-707 #done

Change-Id: I5893719e6bb72aaba481ac0a0cbe43911cfea29c