Introduced a new `Logger` trait which can be used to access all log events
emitted during compilation. Currently these only include messages emitted by the
`@debug` and `@warn` statements.
Changes were implemented in a backwards-compatible manner, but the current
`Options::quiet` method has been marked as deprecated, as its behavior can be
achieved using the `NullLogger` structure. The default logger used is
`StdLogger` which writes all log events to standard error. This reflect the
default behavior prior to introduction of `Logger`.
With these new changes, it is also now possible to properly test the `@debug`
and `@warn` statements.
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