The standard logging interface of the Mill build tool.

Contains these primary logging methods, in order of increasing importance:

- debug : internal debug messages normally not shown to the user; mostly useful when debugging issues

- ticker: short-lived logging output where consecutive lines over-write each other; useful for information which is transient and disposable

- info: miscellaneous logging output which isn't part of the main output a user is looking for, but useful to provide context on what Mill is doing

- error: logging output which represents problems the user should care about

Also contains the two forwarded stdout and stderr streams, for code executed by Mill to use directly. Typically these correspond to the stdout and stderr, but when show is used both are forwarded to stderr and stdout is only used to display the final show output for easy piping.