fix(tfhe): add automatic initialization for ciphertexts

[tremblaythibaultl]

This PR removes the following incorrect logic of some binary operators:

if (a == 0) {
  return b;
} else if (b == 0) {
  return a;
}

and introduces initialization logic such that if a 0 handle is passed to any binary operator, the handle is set to point to the trivial encryption of 0.
This makes it so that FHE operations on uninitialized ciphertexts do not revert, even in the case of ciphertext/plaintext operations.

This PR also introduces a fix for a bug regarding ciphertext/plaintext operations and Solidity's overload resolution.
With this fix, ciphertext/plaintext operations are only defined for values of the same bit width, meaning add(euint32 a, uint32 b) is defined but add(euint16 a, uint32 b) is not.
It is worth noting that the overload resolution process will implicitely upcast the uint value such that both

add(euint16 a, uint8 b)
add(euint16 a, uint16 b)

will work but

add(euint16 a, uint32 b)

will not.

[dartdart26]

Looks good!