gtest ~renamed INSTANTIATE_TEST_CASE_P to INSTANTIATE_TEST_SUITE_P and
added a "backwards compatibility" shim, but the shim fails at compile
time if you pass in an empty fourth argument.
The reason things weren't working here is because libsodium does
something completely unintuitive and called the seed the "secret key"
when it isn't, it's the seed.
This adds a new PrivateKey class (alongside the existing SecretKey and
PubKey) that holds just a private key value but no seed -- which we need
to do because there is no way we can get a seed after calculating a
derived keypair.
With these changes, we now generate exactly the same keys and subkeys as
Tor (and a new test case uses values generated in Tor to verify this).
This is incomplete -- the subkey signing code is still not implemented;
it has to be adapted to create a signature from a PrivateKey rather than
a SecretKey which will probably requiring working around/reimplementing
some of what libsodium does for creating a signature since it expects
"secret keys" i.e. the seed.
