** Use to inject config values into your classes. Example:
**   syntax: fantom
**   @Config { id="gzipThreshold" }
**   private Int gzipThreshold
** If 'id' is not provided, it is matched against a combination of field name, class name, and pod name. 
** See user guide for a complete description of the matching strategy. 
facet class Config {
    ** The id of the config value to be injected.
    ** If 'null' then a matching strategy is utilised. 
    const Str? id
    // null is better than a 'def' value because IocConfig can never supply null, therefore there 
    // is never any ambiguity
    ** 'optional' means an Err is **not** thrown if the config cannot be found.
    ** Fields are not set if config could not be found. This lets 'optional' fields take default values.
    **   syntax: fantom
    **   @Config
    **   private Duration? timeToSelfDestruct := 10sec
    ** If 'null' then it defaults to 'true' if the field is nullable.    
    const Bool? optional