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/tree.h

@@ -46,6 +46,7 @@ struct TSTree {
 typedef Array(TSTree *) TreeArray;
 TreeArray ts_tree_array_copy(TreeArray *);
 void ts_tree_array_delete(TreeArray *);
+size_t ts_tree_array_essential_count(const TreeArray *);
 
 TSTree *ts_tree_make_leaf(TSSymbol, TSLength, TSLength, TSSymbolMetadata);
 TSTree *ts_tree_make_node(TSSymbol, size_t, TSTree **, TSSymbolMetadata);