Fix field element encoding for elliptic curve coefficients

Fix field element encoding for elliptic curve coefficients

SEC 1, section 2.3.5, is explicit that the encoding of an element of the
field of definition for an elliptic curve needs to be a zero-padded octet
string whose length matches the byte size of the field's degree. So use
BN_bn2binpad() to fix this. Factor things into a simple helper to avoid
copy-pasting.

This gets rid of some of the most grotesque code in this file.

ok jsing