EdDSA (Ed25519/Ed448) key support.
PKey compatibility improvements, X.509 and SSL fixes.

• [fix] prepend leaf cert to extra_chain_cert (#181)
• [compat] Store#add_cert and add_crl handle wrong argument
• [compat] Store#time= accepts numeric epoch seconds like CRuby
• [refactor] add ASN.1 sequence bounds checks in PKey parsing
• [compat] implement SSLSocket#pending bytes
• [fix] make sure Cert#tbs_bytes is up-to-date (#364)
• [fix] SPKI#sign NPE due missing ASN1 registry fallback
• [feat] implement Netscape::SPKI#to_text
• [compat] PKey::DH#derive + fix compute_key output format
• [compat] expose derive, raw_private_key, raw_public_key on PKey
• [compat] make to_text() available for all PKey classes
• [compat] expose public_to_der, public_to_pem, private_to_der, private_to_pem on PKey
• [feat] implement Ed25519/Ed448 (EdDSA) key support