In OpenSC the EC field_length is the number of bits in the field.
Most curves have a field_length which is a multiple of 8 bits
but there are many that are not.
The X and Y points and privateD are stored in octetstrings
so there may need to be an extra byte in the octetstring.
An OpenSSL BIGNUM will drop leading zero bytes, so its size can not be used
to determine the field_length.
fix#440fix#433
Rework the EC key generation mechanism to send the curve parameters to
the card. In earlier versions, the applet had a copy of the curve
parameters and there was a different algorithm reference for every
curve. This is unfeasible when trying to support a larger number of
curves because of size limitations of the applet.
This commit additionally includes some refactorings that should not
change the functionality.