Update defaults for variability of pointed-to types.

Now, if rate qualifiers aren't used to specify otherwise, varying
pointers point to uniform types by default.  As before, uniform
pointers point to varying types by default.

   float *foo;  // varying pointer to uniform float
   float * uniform foo;  // uniform pointer to varying float

These defaults seem to require the least amount of explicit
uniform/varying qualifiers for most common cases, though TBD if it
would be easier to have a single rule that e.g. the pointed-to type
is always uniform by default.

tests/new-delete-6.ispc

@@ -7,7 +7,7 @@ struct Point {
 
 export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
     float a = aFOO[programIndex]; 
-    Point * varying buf = new Point(0., b, a);
+    uniform Point * buf = new Point(0., b, a);
     RET[programIndex] = buf->z;
     delete buf;
 }