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.)
* ADD new /src/common/crypto_rand.[ch] module.
* ADD new /src/common/crypto_util.[ch] module (contains the memwipe()
function, since all crypto_* modules need this).
* FIXES part of #24658: https://bugs.torproject.org/24658
This patch moves the pregenerated RSA key logic into a new
testing_rsakeys.c.
Also, it adds support for RSA2048, since the link handshake tests
want that.
Also, it includes pregenerated keys, rather than trying to actually
generate the keys at startup, since generating even a small handful
of RSA2048 keys makes for an annoying delay.
