Why Process.fork needs a good alternative in crystal

@straight-shoota I’m 99% sure that explicit exit ain’t needed (never noticed any failure) and that it’s just me being overly cautious.

Alternatively, one can redefine Crystal.main_user_code to achieve the same effect:

def Crystal.main_user_code(argc : Int32, argv : UInt8**)
  # ...
  previous_def
end