Move some generally useful functions from PER to utility module

[Nicceboy]

I think these functions (log2, to_vec etc.) might be better located on utils.rs or something similar, since they are not codec specific. For example, OER needs them as well.

[XAMPPRocky]

Thank you for your PR! The only thing I would change is that I wouldn't have a single utils module. I don't like those because it's too generic, anything could be put there.

I would instead make two modules num and bits. I would move log2 to num, the rest to bits and rename range_from_bits to range_from_len.

[Nicceboy]

I made the change. Should these have own directory as well or current is good?

[XAMPPRocky]

I don't think we need them to have a directory unless they have sub modules.