feat: compiler warning for using shielded values in branching conditions + prevent shielded types as inputs for abi.encode

[matthias-wright]

This makes two changes to prevent leakage of private information.

1. This adds a compiler warning (code 9663) for using shielded values in branching conditions for the following statements: if conditions, while loops, for loops, and ternary operators
2. Prevents the usage of shielded types as inputs for abi.encode, abi.encodePacked, abi.encodeWithSelector, abi.encodeWithSignature, abi.encodeCall

The motivation for 1 are examples like the following. In this if condition

  if (shieldedBalance > suint(1000)) {
      // code path that costs 20k gas
  } else {
      // code path that costs 2k gas
  }

the gas usage can give information about the shielded value shieldedBalance.

Tests were added to verify this behavior.

The full warning looks like this:

Warning 9663: (106-115): Using shielded types in branching conditions can leak information through observable execution patterns such as gas costs, state changes, and execution traces.

The motivation for 2 is the fact that abi.encode etc all return bytes. Since bytes is not shielded, it would leak information about the shielded input.