Support supplying conditional expressions (predicates) to actions

[leonardt]

This would be a good first step towards a full-blown monitor interface. The small, but useful example is processing valid signals, where data should only be written out to file when valid is high.

[Kuree]

Does that also cover necessary loop controlling to implement JTAG or AXI-LITE driver? These driver interface usually uses some loop state control such as "while this signal is high" or "while this is signal is low".

[leonardt]

I think we can use the same logic. Basically, we can start with the concept of a conditional expression that can be used to predicate actions or serve as a while loop condition.