Let's design a simple logging utility, using a trait Logger with a log
method. Code which might log its progress can then take an &impl Logger. In
testing, this might put messages in the test logfile, while in a production
build it would send messages to a log server.
However, the StderrLogger given below logs all messages, regardless of
verbosity. Your task is to write a VerbosityFilter type that will ignore
messages above a maximum verbosity.
This is a common pattern: a struct wrapping a trait implementation and implementing that same trait, adding behavior in the process. What other kinds of wrappers might be useful in a logging utility?