Add support for directives

[joaovam]

ChiBench only support modules, in the sense that every directive generated is erased from the final program. We should add the support for directive flags, for example:

• ifdef
• ifndef
• endif

The simpler solution would be to generate two programs, let's call them A and B.

The final program generated would be:

//This define should be randomized, for some programs we should follow the if branch, for others the else branch
`define B1

`ifdef B1
A
`else
B
`endif

This solution would cover only straightforward define cases, in real programs these directives are used with much more granularity, for example, inside functions, modules between statements, and so on.