Add gzip log writing to the ascii writer.

This feature can be enabled globally for all logs by setting
LogAscii::gzip_level to a value greater than 0.

This feature can be enabled on a per-log basis by setting gzip-level in
$confic to a value greater than 0.

src/logging/writers/ascii/ascii.bif

@@ -12,3 +12,4 @@ const empty_field: string;
 const unset_field: string;
 const use_json: bool;
 const json_timestamps: JSON::TimestampFormat;
+const gzip_level: count;