[WIP] Add unencrypted PEM PKCS8 dumping support

Adds unencrypted PEM dumping for private keys. Public keys and encrypted (PBE) support will be in separate PRs (and PBE requires some discussion. I tried to dig up the issue we've used for this in the past but couldn't find it)
In terms of thinking about compatibility, what other libraries would we want to test against? I suppose we want all versions of OpenSSL we support, CommonCrypto, ... what else?