Re #290: Migrate to using key/keyref definitions instead of ID/IDREF type * Simplify "non-existant ability" unit test by removing SystemX file * Remove line number checks, since Mono exception line numbers are end of file when the exception is a missing key ** * Re-implement equipment slot check test ** previous line numbers were taken from errors saying that keyref's key definition couldn't be found, not an error saying that the keyref value didn't exist