Support for unlimited user data size (a.k.a. multi-ciphertext SecureVector/SecureMatrix)

[sloede]

Currently, SecureVector and SecureMatrix store exactly one ciphertext to hold all user data, limiting us to ~65K entries when we have a ring dimension of $N = 2^{17}$. It would be great if we could extend this to (theoretically) arbitrary sizes, by extending the implementation of SecureVector and SecureMatrix to hold an array of ciphertexts (instead of a single one). This would also require an extension of the circshift implementations for secure vectors/matrices.