Also, add a stubbed-out nss version of the modules.  The tests won't
pass with NSS yet since the NSS modules don't do anything.

This is a good patch to read with --color-moved.

This cleans up a lot of junk from crypto_rsa_openssl, and will
save us duplicated code in crypto_rsa_nss (when it exists).

(Actually, it already exists, but I am going to use git rebase so
that this commit precedes the creation of crypto_rsa_nss.)

These functions exist only to expose RSA keys to other places in Tor
that use OpenSSL; let's be specific about their purpose.

It is not nice to expose a private key's contents without having the
function name advertise the fact.  Fortunately, we weren't misusing
these yet.

(We do this unconditionally, since we still need it for tortls.c)

Notably, there's a test to make sure that it round-trips with
OpenSSL, if OpenSSL is enabled.

We only ever need this to get us a DH ephemeral key object,
so make a function that does just that.

This is comparatively straightforward too, except for a couple of
twists:

   * For as long as we're building with two crypto libraries, we
     want to seed _both_ their RNGs, and use _both_ their RNGs to
     improve the output of crypto_strongest_rand()

   * The NSS prng will sometimes refuse to generate huge outputs.
     When it does, we stretch the output with SHAKE.  We only need
     this for the tests.

This was a fairly straightforward port, once I realized which layer
I should be calling into.