That way we can better report errors which relate to illegal
parameters being detected.

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.

This reverts commit c801a15bca9ea8f3f7abd4be48bebd36c54eeba2.

That way we avoid issues like #118 which are caused by killing the child
process, and we also avoid deadlocks by making sure that recv will terminate
after a long delay.

That is, do not kill the child, but instead switch the roles of
child and parent, and add a timeout on recv to avoid infinite delays.

Relates: #118