Resolve assignments with equal strengths

[RRozak]

Adds step of dynamic signal strengths resolution.
When the strength is symmetric (the same strength is related to both 0 and 1 values), the exact value of the RHS doesn't matter. It only matters if it equals z or not.
Such assignments are handled by changing the values to z of these bits that are overwritten by stronger assignments. Then all assignments can be aggregated as they would have equal strengths (by | on them and their __en expressions). To change the value to z, the RHS should be & with negation of __en expression of stronger assignments. Changing RHS's __en expression is not needed, because it will be then aggregated with __en expression of stronger assignments using |, so & with the negation can be safely skipped.
So the values of overwritten bits are actually changed to 0, which doesn't affect stronger assignments, because | operation was used.
This handling is implemented in the following way:

• group the assignments by their strengths,
• handle assignments of the same strength as they are handled now
• assign results to var__strength and var__strength__en variables
• aggregate the results:
  orp = orp | (var__strength & ~enp)
  enp = enp | var__strength__en,
  where orp is aggregated value and enp is aggregated __en value.