support for vm.toString()

[daejunpark]

motivation:

the bound cheatcode for signed integers uses vm.toString() when printing the result value. e.g., this test failed due to that.

spec:

https://github.com/foundry-rs/forge-std/blob/60acb7aaadcce2d68e52986a0a66fe79f07d138f/src/Vm.sol#L1236-L1252

    /// Converts the given value to a `string`.
    function toString(address value) external pure returns (string memory stringifiedValue);

    /// Converts the given value to a `string`.
    function toString(bytes calldata value) external pure returns (string memory stringifiedValue);

    /// Converts the given value to a `string`.
    function toString(bytes32 value) external pure returns (string memory stringifiedValue);

    /// Converts the given value to a `string`.
    function toString(bool value) external pure returns (string memory stringifiedValue);

    /// Converts the given value to a `string`.
    function toString(uint256 value) external pure returns (string memory stringifiedValue);

    /// Converts the given value to a `string`.
    function toString(int256 value) external pure returns (string memory stringifiedValue);

the semantics of toString(bytes) isn't clear, but the others should be straightforward.

[0xkarmacoma]

the problem is that bound really shouldn't be used in halmos, but in general the vm.toString() methods could be useful

for toString(bytes), we could first try to parse it as a utf-8 string, and if that fails return a hex string