Linker: Eliminate uncalled functions, because they can be ill-defined.

Fixes issue #610. Also provides a testing option to keep uncalled functions.

Test/baseResults/110scope.vert.out

@@ -154,20 +154,6 @@ ERROR: node is still EOpNull!
 0:8                1.000000
 0:11      Branch: Return with expression
 0:11        'a' (in int)
-0:25  Function Definition: cos(f1; (global float)
-0:25    Function Parameters: 
-0:25      'x' (in float)
-0:27    Sequence
-0:27      Branch: Return with expression
-0:27        Constant:
-0:27          1.000000
-0:29  Function Definition: radians(b1; (global bool)
-0:29    Function Parameters: 
-0:29      'x' (in bool)
-0:31    Sequence
-0:31      Branch: Return with expression
-0:31        Constant:
-0:31          true (const bool)
 0:34  Sequence
 0:34    move second child to first child (temp int)
 0:34      'gi' (global int)