In order for public/private key serialization to work correctly we must
store the size of the key because P-521 cannot be stored as full words
inside `UnsignedBigInteger` and therefore is exported as the wrong
length (68 instead of 66).
This makes it also possible to refactor some methods and cleanup
constants scattered around.
Gets almost all import/export tests, expect the JWK ones that calculate
the public key on export. The `SECPxxxr1` implementation currently fails
to do calculations for P-521.
