Allow expressions in constant declarations

Make this work:

```pascal
Const
  Max3d6 = 3 * 6;
  OneThird = 1 / 3;
  Radius = 12.0;
  Area1 = Pi * Radius * Radius;
  Area2 = Pi * Sqr(Radius);
  SecondsPerDay = 24 * 60 * 60; { 86400 }
  Sqrt2 = Sqrt(2);
  Sin45 = Sin(45 * Pi / 180);
  A = True;
  B = False;
  C = A And B;
```

NB: No function calls allowed?