By: ValentinHirschi <valentin.hirschi@gmail.com>
extend rustred tensor mode to multi-loop vacua User request: Continue as planned, but make sure that this new `rustred` mode of `vakint` is invoking a generic efficient tensor reduction actually *implemented in rustred* (as it may be useful to rustred as well). To have it efficient, get inspiration from how it is done in FORM, and also in rustred have two lanes, one efficient one specialised to single scale vacuum topologies (no external shift in denominators, which is what is covered by the existing form tensor reduction in vakint currently), and one generic one (whose implementation can be delegated to later, but whose stub must be present from the beginning).