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This allows us to determine the size of signatures produced by a public key.
Incidentally, this also allows us to determine the size of an RSA key's modulus.

Closes #125

* ed gen, sign, verify works. tested25519 fails
* Ignore some unit tests in ED since it relies on GoLang lib to support Ed keys
* Cleanup sign and verify operations
* Cleanup and prettify
* cleanups
* Conditional X_EVP_PKEY_ED25519 and  X_EVP_Digest{Sign|Verify}[Init] based on openssl version.

Fix incompatiblities with OpenSSL 1.0.x

- created a uniform shim layer for the Go code to invoke that does the
  right thing depending on the version of openssl. functions that have
  yet to be moved can be done so as needed.

- fixed the incorrect use of X509_STORE_CTX_get_app_data in verify_cb to
  pull out the SSL object from the x509 store context, which causes
  crashes when the SSL object index happens to not be zero.

- fix engine not being passed to HMAC_Init_ex

- fix HMAC tests not checking for failures with NewHMAC

Fixes #62

in later openssl, the key param in EVP_PKEY_assign changed from char* to
void*. causes this error:
../github.com/spacemonkeygo/openssl/key.go:324: cannot use
(*C.char)(unsafe.Pointer(rsa)) (type *C.char) as type unsafe.Pointer in
argument to _Cfunc_EVP_PKEY_assign

Newer versions of Go (at least 1.4) do not like mixing C pointer
types. Cast an unsafe.Pointer to *C.char to make the compiler see
matching types.

NID is also now an exposed type

Change-Id: I4642fb9b97256ec6b3ffbe403272efc690bf1606

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