Revert "Remove the separator characters construct"

This reverts commit 5cd07648fd.

The separators construct is useful as an optimization. It turns out that
constructing a node for every chunk of whitespace in a document causes a
significant performance regression.

Conflicts:
	src/compiler/build_tables/build_lex_table.cc
	src/compiler/grammar.cc
	src/runtime/parser.c

src/runtime/stack.c

@@ -50,26 +50,7 @@ size_t ts_stack_right_position(const TSStack *stack) {
   size_t result = 0;
   for (size_t i = 0; i < stack->size; i++) {
     TSTree *node = stack->entries[i].node;
-    result += node->size;
+    result += ts_tree_total_size(node);
   }
   return result;
 }
-
-TSTree **ts_stack_pop_extras(TSStack *stack, size_t *count) {
-  size_t first = stack->size;
-  while (first > 0 && ts_tree_is_extra(stack->entries[first - 1].node))
-    first--;
-
-  *count = (stack->size - first);
-  if (*count == 0)
-    return NULL;
-
-  TSTree **result = malloc(*count * sizeof(TSTree *));
-  for (size_t i = 0; i < *count; i++) {
-    result[i] = stack->entries[first + i].node;
-    ts_tree_retain(result[i]);
-  }
-
-  ts_stack_shrink(stack, first - 1);
-  return result;
-}