Make the dir-key-crosscert element required

In proposal 157, we added a cross-certification element for
directory authority certificates. We implemented it in
0.2.1.9-alpha.  All Tor directory authorities now generate it.
Here, as planned, make it required, so that we can finally close
proposal 157.

The biggest change in the code is in the unit test data, where some
old hardcoded certs that we made long ago have become no longer
valid and now need to be replaced.
Tweak #10162 documentation a bit
Merge branch 'finish_prop157'
Fix crypto/digests test