ec: Add EcKey affine_coordinates

In OpenSSL, new functions were introduced[1] to get/set the affine
coordinates of a point, where OpenSSL itself determines what version of
the _GFp or _GF2m underlying function to call.
This makes that function available.

[1]: https://github.com/openssl/openssl/commit/8e3cced75fb5fee5da59ebef9605d403a999391b



Signed-off-by: Patrick Uiterwijk <patrick@puiterwijk.org>