ast+datum: move a branch of r5js.Datum#fixParserSensitiveIds to Identifier.
datum: refactor r5js.Datum#fixParserSensitiveIdsLambda_.
datum: refactor r5js.Datum#fixParserSensitiveIdsDef_.
datum: move payload_ from r5js.Datum => r5js.ast.SimpleDatum.
misc: remove r5js.PayloadType typedef.