remove History.mask to fix masking="none" issues

This MR removes the mask as decided in #199 (closed) and fixes #222 (closed).

The removal of the mask makes functions annotated with masking="none" and masking="field" consistent. With this MR in place, there is (most likely) no need to change a function body, when the function annotation is changed from "none" to "field" (and the other way round) and opens up the possibility to dismiss the clearance of flags between function calls (an implementation draft can be found here). Especially the last point seems very appealing to me as this would reduce code, but more so conceptual (accidental ?) complexity of the system.