Implement new, simpler function overload resolution algorithm.

We now give each conversion a cost and then find the minimum sum
of costs for all of the possible overloads.

Fixes issue #194.

tests/func-overload-max.ispc

@@ -0,0 +1,12 @@
+
+export uniform int width() { return programCount; }
+
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float a = 1. / aFOO[programIndex]; 
+    RET[programIndex] = max(0, a); 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1. / (1+programIndex);
+}