Old implementation was not really fitting the updating needs and had a
bug making symbols disappear if they haven't changed but their parent
did (e.g. when a C++ constructor's signature changed).

New implementation does:
1) walk old tree, updating or removing rows;
2) add remaining tags.

It walks less than (new_tags + old_tags + new_tags) in the worst case,
thanks to some hash table-based caching;  and also gets rid of the
"valid" column in the symbols tree, saving a few bytes in memory.

Finally, there is a ~7% performance gain (from 21 to 18ms) upon common
tree updates, sometimes more.