Add .Num variable to the Parameter object for templates.

Sometimes, a custom template may like to have indexes for the parameters. I have two specific use cases:
- I was trying to store test cases as tuples, to access tuples I need get<2>(tuple).
- In tester functions that need the parameters saved in variable names, p0,p1,p2,... is a bit useful because then you do not need to prefix all variable names by __ to avoid conflicts.