Distinguish between dereferencing pointers and references.

We now have separate Expr implementations for dereferencing pointers
and automatically dereferencing references.  This is in particular
necessary so that we can detect attempts to dereference references
with the '*' operator in programs and issue an error in that case.

Fixes issue #192.

tests_errors/deref-4.ispc

@@ -1,4 +1,4 @@
-// Illegal to dereference non-pointer or reference type "varying float"
+// Illegal to dereference non-pointer type "varying float"
 
 float func(float a) {
     *a = 0;