Add possibility to customize contents of `pre_randomize`

[tudortimi]

The constraints_utils macro has to implement pre_randomize(), in order to insert constraints. There are cases, though, where the user would like to also add code to pre_randomize(). Something like the following might be ok:

`constraints_utils_begin(some_type)
  `constraints_utils_pre_randomize_begin
    // statement 0
    // statement 1
  `constraints_utils_pre_randomize_end
`constraints_utils_end

The statements between the pre_randomize_begin/end macros should be added to the implementation of pre_randomize.

[tudortimi]

One question that remains is how to handled super.pre_randomize(). Shall the default contents of pre_randomize_begin/end contain a call to super.pre_randomize()?

[tudortimi]

Actually, it might be a better idea to stop using pre_randomize() all-together. We might be able to do it by creating a dummy rand object inside the class and using that object's pre_randomize() as a callback to handle global/instance constraints.

[tudortimi]

Fixed in #11