Regenerate symbols if parents have changed

The generate-symbols.js task runs JSDoc on source files.  Because this takes a long time (13s) to run on the whole library, the resulting symbols file includes additional metadata to make it possible to do incremental symbol generation on subsequent runs.  The 'path' and 'extends' metadata for a symbol are used to determine what needs to be regenerated.
Call generate-symbols.js from build.py