nettle: use rsa_*_key_prepare on key import

Previously we calculated the size of the key directly, but
by using the rsa_*_key_prepare we benefit from any checks that
may be introduced in the future. Specifically any checks for invalid
public keys (e.g., keys that may crash the underlying gmp functions).

This patch avoids calling rsa_private_key_prepare every time we construct
a nettle private key struct, because this function requires a bigint
multiplication. We call that function once on private key import.