Tm/fix signed decomposer

[tmontaigu]

Some parts of the code did not use the correct way to
decompose a clear integer into blocks which could be encrypted
or used in scalar ops.

The sign extension was not always properly done, leading for example
in the encryption of a negative integer stored on a i8 to a
SignedRadixCiphertext with a num_blocks greater than i8 to be incorrect:

let ct = cks.encrypt_signed(-1i8, 16) // 2_2 parameters
let d: i32 = cks.decrypt_signed(&ct);
assert_eq!(d, i32::from(-1i8)); // Fails

To fix, a BlockDecomposer::with_block_count function is added and used
This function will properly do the sign extension when needed