Where possible, prevent Magic numbers

[MoonLabsDev]

packages/vm/core/evm/evmimpl/iscmagic_state.go contains the magic number MAX_UINT256.

for token approval it is only possible to approve anything <= MAX_UINT64. There is 1 exception, which is that if the value is == MAX_UINT256, then ithe approval amount is clamped to MAX_UINT64. That means that it became a magic number and all values > MAX_UINT64 && values < MAX_UINT256 you throw an error. To remove the magic number and make it cleaner, just clamp everything above MAX_UINT64 to MAX_UINT64 as it all means approve as much as posible. This also makes the code easier:

OLD:

if !numTokens.IsUint64() {
    // Calling `approve(MAX_UINT256)` is semantically equivalent to an "infinite" allowance
    if numTokens.Cmp(gethmath.MaxBig256) == 0 {
        numTokens = big.NewInt(0).SetUint64(math.MaxUint64)
    } else {
        panic(errBaseTokensMustBeUint64)
    }
}
allowance.BaseTokens = numTokens.Uint64()

NEW:

if !numTokens.IsUint64() {
    // Calling `approve( > MAX_UINT64)` is semantically equivalent to an "infinite" allowance
    numTokens = big.NewInt(0).SetUint64(math.MaxUint64)
}
allowance.BaseTokens = numTokens.Uint64()

Also it prevents unexpected behaviour for smart contract developers.