Return a TreeArray from ts_stack_pop

Since the capacity is now included in the return value, the buffer
can be reused in the ts_parser__accept function. Also, it's just
cleaner to use Array consistently, rather than a separate buffer
and size.

src/runtime/stack.h

@@ -16,9 +16,10 @@ typedef struct {
   TSLength position;
 } StackEntry;
 
+typedef Array(TSTree *) TreeArray;
+
 typedef struct {
-  TSTree **trees;
-  size_t tree_count;
+  TreeArray trees;
   int head_index;
 } StackPopResult;