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.

stmt.cpp

@@ -2426,7 +2426,7 @@ lProcessPrintArg(Expr *expr, FunctionEmitContext *ctx, std::string &argTypes) {
         return NULL;
 
     if (dynamic_cast<const ReferenceType *>(type) != NULL) {
-        expr = new DereferenceExpr(expr, expr->pos);
+        expr = new RefDerefExpr(expr, expr->pos);
         type = expr->GetType();
         if (type == NULL)
             return NULL;