KeyVariant type needs to be available in API. Instead of using
PrimeFieldId we can simply use uint8

Those two constnts are not used anymore and can be safely removed
as it is not used anymore. SidhParams struct was also modified.

Private key generation can take advantage of the fact that keyspace for
secret key is (0, 2^x - 1), for some possitivite value of 'x' (see SIKE,
1.3.8). It means that all bytes in the secret key, but the last one, can
take any value between <0x00,0xFF>. Similarily for the last byte, but
generation needs to chop off some bits, to make sure generated value is
an element of a key-space.
Assuming uniform distribution of bytes generated by RNG, secret key is
still chosen uniformly at random, but there is no need to maintain field
specific assembly code.