Add support for EdDSA keys

Features list:

• PKI:
  • certificate import, export, generation
  • private key import, export, generation
  • verification of signatures in trust chains
• PKCS7:
  • verification of signatures
  • signing the data
• TLS 1.2 key exchange signing

Testsuite:

• basic eddsa certificate/key tests
• PKCS7 sign/verify (cert-tests/pkcs7)
• TLS/DTLS key exchange checks ( *tls1.*-cert-key-exchange.c)
• OID functionality tests (oids.c)
• Plain signature/verification tests (x509sign-verify2.c)
• key import/export test (key-import-export.c)
• interop checking with openssl (suite/testcompat-main-openssl) (#228)
• manual interop tests
• TLS with boringssl
• certificate signatures
• PKCS#7 signatures (#227)

As no released version of openssl supports EdDSA, nor any test vectors for PKCS#7 signatures exist, the remaining items are issues to be handled prior to release (#227, #228).

Resolves #25 (closed)