Simp can now act on (f_a)^4 (treated as f_a^2 f_b^2) and (f_a g_a)^2 (treated as f_a g_a f_b g_b).

Now there are two processing stages for Simp:
Simp -> simpRaw0 -> simpRaw. The first stage do the above change for power, and the second stage deals with Exp, Sin, ...

Corresponding tests added.