Fix UB

[coolreader18]

I sorta just rewrote it, but there shouldn't be any UB in this

[coolreader18]

Also, do you want to move this to the RustPython org? Do you have permission to create new repos?

[coolreader18]

Actually, I'm not sure if the approach of storing it in the least-significant bit of the pointer will work, since u8 has an align of one and allocators written in rust (e.g. for wasm) might actually pay attention to the layout requested for allocation. Maybe we could bitshift the length left one, since the max length of a slice is isize::MAX: https://stackoverflow.com/questions/32324794/maximum-size-of-an-array-in-32-bits