WIP: Tag inference
This is the prototype for #16970.
The Analysis is implemented in StgInferTags. I do not expect the code in StgInferTags to undergo major changes and tried to document it reasonably well. Feedback on both the code itself as well as it's documentation are welcome.
There are other changes in this branch which, for the most part, I encourage people to ignore at this point.
The analysis itself is complete insofar as it works and get's decent results. But there are still a few obvious improvements left to make.
This MR is mostly so that those interested can give feedback.