Add explicit assert to get_permutated_index

[paulhauner]

What

Add an explicit assertion that the list size is within the bounds of the shuffler.

Why

There is an implicit assert in int_to_bytes4 but I don't think it' immediately obvious.

To find it you would need to be thinking about how Python handles x.to_bytes(len, ..) when x >= 2**(len * 8).

Also, it illustrates the bounds of the algorithm.

Additional Info

You could assert SHUFFLE_ROUND_COUNT < 2**8 for int_to_bytes1 but it's a constant so I omitted that.

[JustinDrake]

Can we go up to list_size <= 2**40 (4 bytes + 1 byte from the // 258).

[paulhauner]

Oh yeah, sorry my bad.

I did 2**32 / 256 == 2**24 instead of 2**32 * 256 == 2**40.

I'll change it now