`Logger` is the new infrastructrure for debug messages. They are
supposed to be used by just creating a global variable in a translation
unit and then writing there directly as if they were a stream.

Key features:

* Self-registration: a `Logger` by default register itself automatically
  in a register. This means that at run-time, unlike with `DBG`, we have
  a list of all the possible `Logger`s. One benefit of this approach is
  being able to list all the available `Logger`s in `--help`.
* Indentation: `Logger`s can be indented. In particular the
  `LoggerIndent` helper class can automatically indent a certain
  `Logger` during the lifetime of its instance.
* Atomic messages: a message is no longer simply delimited by a "\n": to
  mark the end of a message, an instance of `LogTerminator` has to be
  streamed to the `Logger`. This can also be associated directly to the
  lifetime of an object by using the `LogOnReturn` class.
* Custom class formatting: it is possible to decide how a certain object
  should be formatted when sent to a `Logger` by implementing the
  `writeToLog` function. For example, `llvm::Value`-derived objects are
  passed through the `getName` function. If no custom formatter is
  specified, the `operator<<` is employed.