Merge equivalent stacks in a separate stage of parsing

* No more automatic merging every time a state is pushed to the stack
* When popping from the stack, the current version is always preserved

src/runtime/array.h

@@ -43,7 +43,7 @@ extern "C" {
 
 #define array_push(self, element)                                       \
   (((self)->size < (self)->capacity ||                                  \
-    array_grow((self), (self)->capacity ? (self)->capacity * 2 : 4)) && \
+    array_grow((self), (self)->capacity ? (self)->capacity * 2 : 8)) && \
    ((self)->contents[(self)->size++] = (element), true))
 
 #define array_splice(self, index, old_count, new_count, new_elements)          \