[SoA backend]: Findwitnesses is broken after changing the type of Free Variables in the compiler

[vidsinghal]

The following commit breaks find witnesses: 0352e96...13e0c1d

This commit was added for the following reasons:

LocVar is a complex definition not just type LocVar = Var
RegVar needed to catch up so the definition changed from type RegVar = Var

The allFreeVariables gets us Var, Locs and RegVars since these were all vars it wasn't much of a problem.

Now I had to make a new type FreeVarsTy which is returned by allFreeVars
However, This breaks the functionality of Find Witnesses.
This means that somewhere in the pass it doesn not differentiate Locs, variables and regions which is a fundamental problem.

We should correct its functionality.

I ran the following compilation command in the examples directory:
gibbon --packed --no-gc --to-exe -v4 MonoTree.hs

Maybe freeVar should still return vars but

[vidsinghal]

I think i fixed this in this commit: 6bfad7f

gFreeVars were returning locations and I removed that.
Someone should verify this though