Merge: Introduce `Logger`, a simple yet powerful logging system

Merge: Introduce `Logger`, a simple yet powerful logging system

# A simple logger for Nit

## Basic Usage

Create a new `Logger` with a severity level threshold set to `warn_level`:

~~~nit
var logger = new Logger(warn_level)
~~~

Messages with a severity equal or higher than `warn_level` will be displayed:

~~~nit
logger.error "Displays an error."
logger.warn "Displays a warning."
~~~

Messages with a lower severity are silenced:

~~~nit
logger.info "Displays nothing."
~~~

`FileLogger` can be used to output the messages into a file:

~~~nit
var log_file = "my.log"

logger = new FileLogger(warn_level, log_file, append = false)
logger.error("An error")
logger.info("Some info")
logger.close

assert log_file.to_path.read_all == "An error\n"
log_file.to_path.delete
~~~

## Severity levels

Each message is associated with a level that indicate its severity.
Only messages with a severity equal to or higher than the logger `level`
threshold will be displayed.

Severity levels from the most severe to the least severe:

* `unknown_level`: An unknown message that should always be outputted.
* `fatal_level`: An unhandleable error that results in a program crash.
* `error_level`: A handleable error condition.
* `warn_level`: A warning.
* `info_level`: Generic (useful) information about system operation.
* `debug_level`: Low-level information for developpers.

## Formatting messages

You can create custom formatters by implementing the `Formatter` interface.

~~~nit
class MyFormatter
	super Formatter

	redef fun format(level, message) do
		if level < warn_level then return message
		return "!!!{message}!!!"
	end
end
~~~

See `DefaultFormatter` for a more advanced implementation example.

Each Logger can be given a default formatter used to format the every messages
before outputting them:

~~~nit
var formatter = new MyFormatter
var stderr = new StringWriter
var logger = new Logger(warn_level, stderr, formatter)

logger.warn("This is a warning.")
assert stderr.to_s.trim.split("\n").last == "!!!This is a warning.!!!"
~~~

Optionally, a `Formatter` can be given to replace the `default_formatter`
used by default:

~~~nit
# Create a formatter with no default decorator
logger = new Logger(warn_level, stderr, null)

# Display a message without any formatter
logger.warn("This is a warning.")
assert stderr.to_s.trim.split("\n").last == "This is a warning."

# Display a message with a custom formatter
logger.warn("This is a warning.", formatter)
assert stderr.to_s.trim.split("\n").last == "!!!This is a warning.!!!"
~~~

Pull-Request: #2751
Reviewed-by: Jean Privat <jean@pryen.org>