Add assert() statement support. Issue #106.

tests/assert-1.ispc

@@ -0,0 +1,15 @@
+
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    float a = aFOO[programIndex]; 
+    // Neither of these should hit!
+    assert(a > 0);
+    assert(b == 5.);
+    RET[programIndex] = 2*a; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 2 + 2*programIndex;
+}