* Clean up unused EC bindings.
A lot of these are really OpenSSL internals, like the EC_METHOD
business, support for custom curves which are a bad idea, and weird
non-standard serializations like taking the usual point serialization
and treating it as a single BIGNUM.
I also didn't remove things when they're arguably part of a set. E.g.
EC_POINT_add is used, but EC_POINT_dbl isn't. However, they both set at
the same abstraction level (basic point operations), so it's strange to
have one without the other.
I also kept EC_POINT_is_on_curve because, although it is not used,
OpenSSL prior to 1.1.0 doesn't perform this important check in
EC_POINT_set_affine_coordinates_GFp (though it does in some of the
functions which ultimately call it, like
EC_KEY_set_public_key_affine_coordinates, what cryptography.io actually
uses), so one should not expose the latter without the former.
* Fix build issue.