Generalize/improve parsing of pointer declarations.

Substantial improvements and generalizations to the parsing and
declaration handling code to properly parse declarations involving
pointers.  (No change to user-visible functionality, but this
lays groundwork for supporting a more general pointer model.)

tests/funcptr-null-1.ispc

@@ -8,7 +8,7 @@ float foo(float a, float b) {
 }
 
 static float bar(float a, float b) {
-    return min(a, b);
+    return a<b?a:b;
 }
 
 export void f_f(uniform float RET[], uniform float aFOO[]) {