Fix bugs with handling types of integer constants.

We now follow the rule that the type of an integer constant is
the first of int32, uint32, int64, uint64, that can hold the
value.  (Unless 'u' or 'l' suffixes have been provided.)

Fixes issue #299.

parse.yy

@@ -144,7 +144,7 @@ struct ForeachDimension {
 %}
 
 %union {
-    int64_t intVal;
+    uint64_t intVal;
     float floatVal;
     std::string *stringVal;
     const char *constCharPtr;