Adding binding for void *X509V3_EXT_d2i(X509_EXTENSION *ext);

Can we add bindings for this function so that in pyopenssl we can grab the translated X509 structure according to its extension type. Other wise we have to do cruft like _lib.X509V3_EXT_get(self._extension) to obtain function pointers to the allocation, d2i and invoke those function pointers. It looks like some pyOpenSSL guys are mimicing the method calls made by X509V3_EXT_d2i in absense of it.
Per our style guide the function argument should just be `X509_EXTENSION *`. Otherwise this LGTM assuming it passes the jenkins gauntlet of OpenSSL versions.