Test that grammars.yml agrees with the list of submodules

If any submodules are missing from grammars.yml, or are listed in
grammars.yml but missing from the repo, the test will fail.

Eventually it would be good to test that the scopes for each submodule
are accurate, but that will take some more work.
Suggest running script/bootstrap before building gems

This ensures all our submodules are up to date as well.
Update submodules in parallel to speed up bootstrap

This runs 8 `git submodule update` processes in parallel, speeding up
bootstrap from 2 minutes to 30 seconds for me. (Obviously this is
dependent on bandwidth.)
Initialize submodules before updating them

Initialization cannot happen in parallel because every submodule has to
get written to .git/config.