Add functions to encode unsigned integers as `i64`

[Janmajayamall]

For example

fn encode_unsigned_to_i64(data: &[u64], k_pt: usize) -> i64

The function first maps each unsigned value to signed representation in plaintext modulus and then casts to i64. This is different from directly casting unsigned integer to signed integer.

[Janmajayamall]

At the moment, encoding and decoding output inconsistent values (after any homomorphic operation). And it's a bit confusing why

Edit1: As a consequence, glwe/test_fft64/encryption.rs/test_encrypt_sk fails when k < 64

Edit2: Probably we'll have to add decode_i64_to_unsigned as well.